I'm sure it doesn't just happen to me alone. Mobs seems to gain some incredible evasion bonus when they are at >50% hp. Often time I would wiff wiff wiff, use WS....wifff...and wiff some more. My best record was 10 missed in a roll on a blue mob (that still gives decent sp) when it was at 10% hp or so. While I was busy missing, the mob had no problem landing every single hits including 4 WS one after another and have me licking dirt with the mob dancing on my flattened Lallafeh.
I hope this is a bug but knowing SE...this can very well be working as intended :P