Poker bots?

[Vedocorban]

I frequently play head up freeze-outs online, since I believe it suits my skill set well. However, every so often, I get the feeling I'm playing against a computer program. Perhaps it's just different people using similar styles, I don't know. But they -rarely- show any aggression when I play them, and when they do it's usually the minimum bet or an exact pot sized bet, and they -never- raise preflop. I've even seen them check an unraised pot down to the river, holding AQ when an A flops. Maybe I'm just frustrated with my lack of results against such players (1 out of 3), but I've been beaten before by people who seem to have some imagination and thought nothing of it. Am I being paranoid here, and these people are actually just playing very solid, mathematical poker... or is this a valid concern? If so, are online cardrooms doing anything to prevent coded robots from mingling with their human patrons?