Yeah, this is a bit disconcerting but, here goes:

playing roche's path in chapter 2 and stumbled upon an arachas, decided to run away and

tackle the creature later. But for a while now (3 hours so far), No sign of the creature anywhere. I returned again and again, but the damn insct won't spawn. Meditated for hours :no change.

Ok, later on I stumbled upon a bullvore in a cave (luckily I did a quick save before the fight), but I got owned. Did a quick load, but the bullvore won't spawn again.

Is this a feature in the game? or a bug? I really want to kill that bullvore!
oh, and for some odd reason, sometimes pressing "R" immediately locks up the game. I can only run in circles and do nothing else.

They spawn randomly.

This is a known bug that has been discussed in several other threads... we are hoping it will be fixed in the next patch. I've encountered it several times, and it seems that the only way to get out of it is to let an enemy hit you. It is possible to recover from it without dying but it's annoying.

You may have to come back the next in-game day (or after midnight, at least) to get a respawn. The respawn logic seems to favor not respawning certain enemies for a good long time, then respawning hordes of them all at once.

The "bomb throwing bug" (controls lock up after pressing "R", if there's nothing in the bomb/trap/knife slot) is both annoying and well-known. Some have said getting hit will unstick the controls.

ah, thanks for all the replies guys!. I really love this game.

It can occur even when there IS something in the slot. It's happened to me several times, even after patch 1.3.