DD is now 16 mo. At 13mo, her naps were still all 30 minutes exactly (you could set your watch by her!), and she always woke up crying. I could never soothe her back to sleep, and had tried nearly everything that seemed like it could work.
At 15mo, she suddenly lengthened naps and then dropped down to one 2 hour nap, but only if held throughout and bf back to sleep at transitions. As if today she no longer requires soothing back to sleep!
Not trying to boast, but thought it might be a bit of a light at the end of the tunnel to others with poor nappers. Nothing worked for fixing it, but it just fixed itself!
Caveat - she's still not a great sleeper at night, but for the first time in ages I feel hope that this will all work out without having to resort to methods I'm not comfortable with.