Hi,
My daughter Isabelle never used to nap for long during the day so at about 4months we decided to try and sort it out. It wasn't easy but we used the 'pick up, put down' method to extend her naps. It worked!! She now sleeps for up to an hour in the morning and up to 2 hours (sometimes longer) in the afternoon.
I'm not an expert but it seems as though you're lo is having difficulty making the transition into deep sleep, which is when she wakes up.
I used an excellent website to help me (we recently had to revisit due to some nighttime waking). For £8 you get a book with a summary of the strategies that can be used to help your lo get more sleep plus access to a forum where the books author will give advice on a daily (sometimes more often if needed) basis to help support you through whatever strategy you choose. I can't recommend it highly enough but it's obviously your choice. She will tell you to ditch the dummy though..... If you're interested the site is
www.babysleepanswers.co.uk.
It's likely to be hard work, Isabelle was a tough nut to crack - but for me the outcome was well worth it....
Nicki