How long are you LO's awake before they need to nap?
My LO: I'm finding that even though he sleeps all night, he needs a nap about 1.5 - 2 hours after waking. Somehow if we time it JUST right he'll do 1.5 hour nap and sometimes he's up at that dreaded 45 minute. After that it's just all over the place. Maybe if I stayed home every single day and experimented with the next naps I could figure it out but we go and do stuff, fun or run errands. I always try to time those errands so he'll nap in the car or if we stay home I just kind of watch for those sleepy cues but I seem to miss it and end up with an overtired baby who fights the nap or is pretty cranky at bedtime. I think as the day goes on he can have a longer wake time but I'm still finding it hard to catch him before he's overtired and fighting it.
Re: 5 month old naps
My LO is also 5 months and he needs to go down for a nap after 2 hours. I always start the process of putting him down after it's been about an hour and a half. I have learned that if I put him down a little bit earlier than I think I should, it makes a big difference. My LO will sometime fuss when I lay him down but when it happens it's under 2 minutes of fussing. When I would wait to put him down closer to 2 hours, he would get overtired and fight it really bad. Maybe try putting him down about 30 minutes before you normally would and see if that makes a difference.
I did read that at this age it's normal for babies to stay awake for 2-3 hours at a time. I have noticed my LO doesn't show signs of being tired until closer to the 2 hour stretch, so the past couple days I have been putting him down after it's been 2 hours and I haven't had any issues. He also has started eating more so I don't know if that is making a difference.
Bottom line is right now your LO may just need to go down for a nap closer to 1.5 hours. I would definitely put him down earlier than you normally would and see if it helps. Good luck!