Not capturing movement for hours at a time

My cameras(3)May work fine for days or weeks then all of a sudden they quit capturing movement for hours at a time.   This has been an ongoing problem ever since I have installed them which was a couple years ago.   I have done everything Tend suggest but nothing works?    They will start working normally on their own after several days which leads me to believer it is a Tend Server problem?   Really like the little cameras but they are useless if you can’t depend on them!