Hit spiders issue

I can't hit spiders- the first hit demages them, but the next one not. The problem is only occurs with spiders, other creatures I can attack normally. The weapon doesn't matter. The problem concers all spiders, from various nests- both natural one and those planted by me. If it could matters- the problem started when I set fire one (natural) nest. Restarting the game doesn't help. No new mods for a days and all current.