What time does she wake up for the morning, and what time are you trying to get her to take a nap? It could be that she is already over tired when you try to put her in, and so she cries more, because she is having trouble settling herself. If you think this might be the case, try putting her in a little earlier and see if that works. My 6 month old wakes in the morning between 730 & 800, then begins to fuss for her nap around 10 or 1030. I find she can't handle being awake for much more than 2 to 2 1/2 hours at a time. Try to observe her for a few days, see if she begins to get a little fussy around 1 1/2 to 2 1/2 hours after waking up. That could be her cue that she is ready to sleep. Maybe feed her a little more, rock her, and put her in her bed. Let her cry for 5 to 15 minutes, see if she can settle herself. I know it is easier said than done, but try to relax. Babies can sense when their moms are stressed, so if you get worked up trying to get her to sleep, it might just upset her more.
It could be too that she is at the age where she is observing so much more, and just wants to know what is going on around her all the time. As she becomes more adjusted to her ever expanding world, she hopefully will settle back into a two nap schedule.
As with everything, you know your baby best. If she seems to function perfectly fine on one nap, let her go. Also, if her first nap is only 30 minutes, but she wakes up cheerful, there is nothing wrong with that either- keep giving her those power-naps. She might not need a longer nap in the morning. Good Luck!