I've had two babies exactly like this. I worried about it with DC1(friends babies would sleep for 1hr30+) and spent a lot of time standing over the cot shushing, patting, etc etc. but it didn't matter in the long run. He was always perky in the day, a reasonable sleeper at night, and by 14months ish he was having one long nap at lunchtime. So when DC2 turned out to be the same way I've not worried about it, and she seems to be turning out fine too!
I think at around 7months DC1 did start dropping one of the naps, with him it was the late afternoon one. It was taking so long to get him to go down I just gave up trying. The other two naps shifted a bit later, then the mid morning one went (again, I just stopped trying to put him down).
One thing I would say is that it's easy to fall into a habit with babies of expecting them to sleep wherever they are, as that's what they do when they're little. As they get older they're more interested in the world and it can be a battle to get them to sleep. When DC1 started dropping naps I had to make a bit more of an effort with the remaining ones to do them "properly", with a dark room, nap time routine etc. This can curtail your activities a bit, so I was relaxed about the mid morning nap but made more of an effort for the afternoon one, as we were generally home then anyway.
As for prolonging the naps, there's a theory that if you make them stir at the right point in their sleep cycle you can reset the cycle and get them to go longer ("wake to sleep") but this never worked for me! IME it is just a developmental thing and they sleep for longer when they're ready. DC1 eventually napped for 2hrs between the ages of 1.5-2.5.