I am a technical support engineer and I have a co-worker in another country whom I need to consult every now and then via Skype chat for software dump analysis (because he is trained on that). Till last 3 years, there was no issue but for last 3 months, he started ignoring me. He never replies to my IMs but I can see him engaged with other people on IMs. So far, I ignored because his work requires concentration and research and I thought he would have been busy. Two days ago, I found that my colleagues at my location chatting with him (for work purpose) and he is constantly replying to them but not to me. They were surprised about this as well.
In one month time my manager is scheduling a training to teach me that skill (it's a promotion too) and the training will be delivered by him only. He also told me that he is generally very strict in his training (as per feedback of previous trainees). This makes me more nervous now.
How should I deal with this? I thought of writing an email but what should I write because I never did anything wrong to him and he may ignore that as well. He is at a different location so I can't walk up to him.