Are those two like that in general (i.e., less grateful and gracious/thankful, on the whole) ? If so, there's your answer. Most likely.
And if not, are they being really disruptive throughout the actual sessions? Or are they just being ungrateful wretches outside the game, and whining a bit here and there? In the first instance, try talking with them about the problem, levelly and firmly, and see if you can get to the bottom of what's eating them*; get them to cough it up, so maybe it can be reasoned through or around. In the second instance, feh, never mind; that they're still turning up to play says a lot, right? Unless your patience/sensitivity can't withstand it of course, and then, you might want to default to the first instance's solution.
It sounds like you're doing well as a DM, either way. Good luck!
* Unless, yeah, as has been mentioned, they are simply powergamers, and you and the others are not. At which point, you gotta decide whether that's cool with you (and the others).