What to do if our supplier is not responding regularly and is not actively involved in our project?
Nobody would really like to have this kind of experience especially when we deal with critical customer. A piece of practical advice is useful: take a look in lessons learned database to see if there was a similar case in the past. At the same time you may want to contact your peers to discuss similar case during their experience and how it was handled. It may also happen that supplier is avoiding communication on purpose, maybe he was not delegated a full responsibility from his project lead.
In any case it is strongly recommended not to guess the reason for his non-involvement but take steps to get back on track as soon as possible.