I guess the first thing you need to tell us is the kind of insulated wire that you are referring to and is this wire being purchased to a proper specification? Likewise have you reviewed that specification for shrinkback requirements?
Generally speaking, insulation shrinkback is a result of inbred stress in the plastic created when the plastic cools in the water bath after extrusion.
Without getting too technical, the two ways shrinkback is reduced are:
(a) By gradient or hot water cooling and/ or,
(b) By cleaning the conductor thereby allowing better adhesion of the plastic to the conductor.
You could also create insulation shrinkback if you accidentally stretched the remaining plastic insulation during the cutting and stripping process.
Assuming that you are sure your process has not created the problem, we recommend that you contact your wire manufacturer and work with them to quickly and permanently resolve this problem.