Hello there!! My DD did the 45 min. nap thing for a while too - I won't tell you how long.
Some of it is just a waiting game until they "learn" to sleep longer. But a few things helped me:
1. The sleep mods. will prob. say not to do PU/PD before 4 months - just too early. They won't understand.
2 .Also, some people have great success with extending the nap but a lot of us don't. I appreciate Tracy's "3 day trial" theory- whatever you try be consistent for at least three days. But if they still haven't caught on and it's just getting frustrating, you might just need to adjust your expectations and find a way to work around it.
3. I finally figured out that my DD was actually OT. I was trying to fit her into a 3.5/4 hour routing when really she needed a much earlier morning nap. I wish now that I had given myself "permission" to play with the routine. You don't want to put them down so early for the first nap that it just becomes an extension of the night, but tbh my DD liked a 2-2.5 morning A time for quite a while. I didn't realize it was "OK" for the A times to be so different. She would do a very short morning A time, a much longer middle time, and then about a medium A time before bed.
All that to say that every LO is different so give yourself permission to play round with things for a bit.
HTH!