I'm going to say no, you can indeed lift more than this. Here's the thing though: it may take a while. Strength comes along in small increments and has to be fought for. It may take the right amount of mental toughness in addition to the right workout to get you there.
If you want to lift heavier and see how far you can go, I suggest the Slow & Heavy weight training DVD set. With S&H, Cathe gives you the right number of reps to allow you to go as heavy as possible and the correct rep speed to allow you to insert and work on the mental toughness needed to reach for the next level of strength. I have never lifted more for the upper body than when training with this set. I normally lift 15 pound dumbbells for 3 sets of military presses, although I usually do Arnold's presses, which I prefer. With S&H I managed to get up to using 20 pounds per arm. I used S&H for about 6 weeks, using it twice through a 10 day rotation to see this improvement. Once you find it hard to push through to the next weight level, this is where plate mates come in handy: little magnetic weights in very small sizes that allow you to make incremental gains to move from 12 pound dumbbells to 15s and from 15s to 20s.
I am a combo of an ectomorph and mesomorph. My arms can be like noodles but I can build muscle if I work the arms and shoulders hard.
How long have you been lifting for? I would say you are nowhere near done yet.
Clare