Hi there,
We struggled with 30 minute naps as well, and I feel your pain when you say you think you're to blame, I felt the exact same way. Some of my days were spent at home all day and all I can remember is trying to make LO sleep and trying to keep her asleep. NOTHING worked. I eventually stopped being so hard on myself but continued to try anyway.
Today at 4.5 months, she taking two 1hr20m naps and 1 (sometimes 2) 45 minutes catnaps in the evening. I'm still hoping they can increase to 1.5hrs but I'll gladly take this!
The 2 things I noticed I had changed around the time her naps increased was, taking the paci away cold turkey ( that was because of multiple and I mean multiple night waking to replug it). And increasing the A time. But not the stimulating A time, more like the wind down A time ( singing a song, reading a book, walking around...) because she still got a little cranky/sleepy at her usual A time.
Not sure if these had anything to do with her extended naps or maybe it just got better as she got older but I just wanted to put it out there in case it helped you any.