Hi there, i'm new here but i too have struggled with the 45 minute nap issue. I have had some success so i'll pass on what worked for me - let me first say that all of the methods where you are actually in the room with baby did NOT work for me, as soon as Anna saw me she broke into a huge smile and wanted to play and there was no going back.
First of all, at about 3-4 months i started really watching for earlier bedtime cues and, no kidding, Anna (now 8mo) was ready for bed at about 5:30. We would start her bath at 5:15 and she would be nursing and ready to go down by 5:30 or 5:45. That shifted with some work on my part to start her bath at 5:30 and we did that for a long time. she was sleeping by 6:00. Then we moved bath to 5:45 and she was asleep by 6:15. I know that this seems to have nothing to do with naps, but i really have found that an earlier bedtime keeps Anna well rested and then naps are easier.
That said, when she consolidated her naps, Anna did develop a 45 minute nap problem. I finally found the solution on this board- her A time was too short. i extended it to 2 hours and 20 minutes and immediately she began sleeping for an hour in the morning and 1h 45 m in the aft, plus a quick catnap before supper. However, just in the last week and a half she went back to 45 minute naps and i posted for the first time yesterday in frustration. Stacy recommended extending A time again and it worked!!! she slept only 30 minutes in teh morning but i think external noise (7 year old bro!) woke her - this aft, with some quiet, she slept 2 hours.
When Anna was taking more naps, i would watch her cues but i find now i need to go more by the clock or else her A time won't be long enough. Also, she tends to cry out at 30 m and 45 m mark but she goes back to sleep within a few minutes. I don't leave her for serious crying - if she starts that i just go get her.
So, bottom line was that for us, a very early bedtime plus extending A time worked. Oh, also, virtually all naps are in her crib, which does limit our ability to go out etc, especially when younger and having 2 hours or less of A time. But i swear it was more than worth it to have a well rested baby!
One more thought - Anna was a lot slower to consolidate her naps (used to nap 4-5 times a day, slept virtually all the time for the first several months) than everyone seems to suggest. So if your lo is not doing what you expect, it may be that he's just a bit slower than others to develop rhythms, like mine.
Good luck, i completely understand your frustration - it can feel so demoralising to not be able to give your lo what s/he needs!
best wishes,
tracey