On some occasions, when I try to rest the HP bars don't bother to fill at all and may take about 5 rests to finally start restoring itself.

Am I doing something wrong or is HP restoration not guaranteed?

Hit points are being restored, but when you have many hit points, the amount restored may not trigger a change in the size of the bar.

The resting algorithm is very weird in W6 and W7.
You need to be active for a while and get tired before you can efficiently rest again.
Would you benefit from four 8-hours sleeps in a row?
Heal HP using healing spells.