The followers tend to lose the best equipment I have given them, is there any way to stop that ?

For some reason, followers re-equip their default gear when you dismiss them. They don't lose what you gave them, they just go back to their default appearance. They'll usually re-equip what you gave them if you ask them to follow you again (sometimes not until you actually get in combat, though), and you can "remind" them to do so by trading items with them again (I usually just take something away and give it back, and this will make them re-equip it all).

Unless you're saying that your followers actually lose things, as in it disappears from their inventory, in which I don't know how to fix that, since it's never happened to me.