Possibly the play, possibly concentration against the opponent. Maybe you are trying to hard to see the board and miss the simple accurate move. I've had the same problem with several players myself
Just Can't Beat Someone

sometimes they just have your number.
try using an opening that You really do not like. that will change the
entire course of your game and force you to concentrate just on the game.

Scissors beat paper, paper beats stone, stone beats scissors :-)
Hehe.
Tal had this very same problem vs Kortchnoi, no wonder that the attacker had problems vs the counterattacker. And let's not forget the psychologi bit in it aswell.
Is it just me or are their people you just can't beat? I've lost several games to people and then easily beat someone that has beaten them.
Is it because our methods are so different and their play makes them better against us?
Chuck