From what i've seen in-game(playing solo), the blocks:

A) Regen normally if they were damaged by common damage(gatlings, rockets, rifles.), haven't tested with non vanilla weapons.

B) Regen at a slower rate some of the damage was caused by a grinder, so to keep the regen pace lower than that of a starter grinder.

C) Stop regenerating at all if the block component status is <40%.

Try checking if your overall damage per mission gets your blocks on 39% or lower.

I never brought the visible damage lower then 50% due to the 50% rule or the engine. But the split between deconstruction und real damage is not shown in the ui so its difficult to say what damage a block really got.

Grinders take from the component pool, so they reduce maximum HP, and normal damage reduce current block HP(edit: that's how i think it works). So basically anything with enough stopping power can kill blocks regardless of regeneration power. If it is focused gatling fire then you're pretty much hopeless, since they can't be denied like rockets can.

Edit 2: Following my logic by components being max HP and damage reducing from current HP, when you lose a certain %, a number of components with corresponding value in the pool are destroyed.