4 wheel drive problem

when i push the 4x4 button on my truck some times it just flashes and it won`t engage in to 4 wheel drive,some times it works fine.i don`t understand what`s going on .Help!!!!!

What year, model, etc. are we talking about???
Check the ele. connection at the front diff.

What year, model, etc. are we talking about???
Check the ele. connection at the front diff.
It is a 1997 gmc k1500 ext.cab pickup with a 5.7 engine

i know the piston on the front diff. does stick sometimes if it is not used often. i try and engage mine at least 1 time every couple of weeks