This is the age where funny things happen to our babies, what with rapid development and growth spurts!! My daughter is just over 4mths and she had the same problem...well, still has but improved somewhat. Before she turned 4mths, she was napping fine even when I was transitioning her to 4hr easy routine. Then all of a sudden, her naps didn't get past the 45min mark (which is one sleep cycle) and like your lo, she'd grin and squeal...cute but a pain when you want her to sleep
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With the 4hr easy, our lo's are supposed to be up for 2hrs from wake up. It was very difficult because my lo would get so tired after only 1hr initially. So I stretched her awake time gradually until now, I wind down 15mins before the 2hr mark. And lo and behold, her naps stretched to 1.5-2hrs. I couldn't believe it! I thought she would be over-tired and stimulated but apparently, being up for the full 2hrs helps her to nap longer even though she becomes a huge crank.
Believe me, it's still difficult to keep her up but somehow she pulls thru. Of course on some days, there should be exceptions but it means you have to be ready deal w/ the short naps. When she's up, I just leave her in the cot and she'll babble and move around until she's tired for the next round of sleep. When she starts fussing and crying, I'll reswaddle her and put her back in and she generally goes back to sleep. Even though it doesn't always work, most of the time she does sleep.
I hope this has helped a little...