Our problem was 30 minute naps. If I got a 45 minute nap, I thought I had hit the jackpot! For us, she overcame the short naps one weekend towards her 5 month b-day. She took a 2 hour nap, mostly because we were traveling and she was tired. That following month they were hit or miss. Some days they were long and some days she snuck back in a 30 minute one. She is now 7 months and I have to say we have finally hit the point where she naps a long time.
I had done everything imaginable. I had taught her how to fall asleep by herself at 4 months. To extend her naps, I spent a week or more going in when she woke after 30 minutes and holding her until she fell back asleep. Most of the time it took 30 minutes to get her back down. Then I would lay her down and she would sleep for 30 minutes longer. It was so frustrating. I feel your pain.
My advice, looking back now (it was a whole 2 months ago), is that you do what you can, but roll with it. If by the 6 or 7 month mark you don't see some improvement than the problem might be more than a nap issue. Maybe a routine issue, learning how to fall asleep independantly, eating, etc. Someone told me time would change it when I was in your position and I didn't believe them. I thought we would have 5-30 minute naps forever...but it did change.