There are two things to address here.
- Whether your suggestions 'should' be accepted
- Dealing with the repetition of this scenario
2 is much more important than 1
tl;dr: step back, be strategic, break the cycle with input from B
Why does this keep happening?
I find the most frustrating things are those that recur without seeming to improve. I quickly see them become Sisyphean and lose heart. But the change does not need to be that you give up and become like all the other peons.
Your suggestions may or may not be appropriate - there is often more to consider than the immediate technical challenge, and often I've been wrong when I didn't know or didn't take into account other considerations. What's more important is progressing from here.
You need to talk to boss B about this repeating scene. Request a meeting with him/her, take them to lunch, whatever. You will need to be calm and receptive, not combative and defensive; your quest is to begin to work with B towards a point where 50% of your suggestions are accepted. This conversation is the only way you will find out whether there are prejudices against your suggestions (e.g. because you are younger, or more junior, etc) - you may not be told it directly, but you will be able to analyse the responses to your questions to see whether there is a plain reason or not.
Should my suggestions be accepted?
At one end of the spectrum, your suggestions could be the optimal solution to the problem in every case, and it is only some blind prejudice or stupidity making the rest of the team disagree. At the other end of the spectrum, your suggestions are all way off, and you are wasting everyone's time. Both of these are unlikely, even though your emotions may want to polarise toward one or the other (it's easier to respond emotionally with either the superiority or inferiority complex).
More likely is some combination. If, like me, you were the junior member of the team but better at seeing solutions to technical problems, then you are probably getting a little discrediting for being junior, but also missing the mark more often than you realise. Other considerations that you may not be taking into account are the YAGNI principle (Ya Ain't Gonna Need It), which is the counter to endless boilerplating, over-complicating things by adding layers of abstraction which won't pay off, or sometimes by assuming that the constraints you are given are more important than the time you spend solving the problem (it's tempting to try to look brilliant by meeting all the constraints, but letting complexity grow significantly in the solution).
This is a learning experience - some of your solutions will be technically wrong, which you will usually learn in the discussion. Some will be wrong from a management perspective - too much complexity, too much time to implement, difficulty of maintenace, reducing future flexibility. Some will be rejected in error. Some will be rejected in prejudice.
Since spending ages going through your suggestions may take a lot of time, particularly if you have a lot of them, I would suggest you ask to meet periodically with just B to improve your approach; in these meetings you will be able to get a more frank statement from B on why suggestions weren't selected, and more importantly get general guidance. You'll note this also gives you an opportunity to show how good some of your solutions are, which may increase the chance of futures ideas being accepted.
Change your approach
I used to be the thirsting upstart, ready to take on the world with my skills. I was rather less amazing than I imagined, but more important was a change in approach. Take everything as a learning opportunity, and try not to react emotionally. It took me a few years to stop taking it personally when my ideas were not accepted, or were shelved for a rainy day.
You need to approach the team strategically. Remember the cardinal rule of employment, you are there to make your boss(es) look good. That doesn't have to be your personal goal, but know that it is the crucial factor in their view of you. So keep detailed notes of your ideas so that you can highlight later (in a private meeting with boss B) how your suggestion would have been appropriate. Maintain a good relationship with boss B and meet with B to work on improving the success ratio; understanding better why your ideas are being rejected will help you both focus on the right things and not suggest things which won't be accepted.
Some answers here are essentially 'you're right, they're wrong' - great for the id, useless in practise. Others are more 'they're right, you're wrong' - great for the ego, and inferiority complexes, and useless in practise. I'm asking you to take a step back, and accept that they're right on some counts, you're right on others, and there's a lot of grey area where neither the situation is much more equivocal. I think this is much more about your relations with the team, and your relationship with yourself. You are not above them, nor should you be quiet.