The main thing you are doing wrong is not using tape to cover the
crack. I would use the mesh type joint tape. Apply that first, then a
coat of drywall compound or one of the similar compounds sold for crack
repair. Those products are supposed to have a little more flexibility
and resistance to cracking.

Some cracks may reopen regardless of what you do. Houses shrink and
expand with changes in temp and humidity. If you set your heat real
low when away for example, then crank it back to 75 when you return,
that can be enough to cause a problem, in some cases.