6 month old naps

My son usually has such a hard time napping and I'm not sure what to do to help him. I've done sleep training at night with the ferber method and he usually sleeps great at night where he's only up once in a 10-12 hour span.

His daytime naps are somewhat less than great. He'll sleep for maybe 30 minutes and then wake up screaming. Usually I'll let him cry for maybe 10ish minutes to see if he'll calm down and go back to sleep. Usually, he does not. So I go get him and we go about our day, but he's obviously still tired because he didn't sleep long enough. Occasionally, I'll get lucky and he'll sleep for an hour or more and he's such a happier baby if he does that. But those days are few and far between. How do I expand his naps? Does anyone have any advice?