I had the same problem until I started trying to put him down before I thought he was tired. Two hours after he gets up, whether that's getting up in the morning or getting up from a nap. He also never slept in a crib before but when I started putting him down for naps in our darkened bedroom in the Pack and Play, he started sleeping for, like, hours at a time, instead of minutes. I had no idea he could sleep so much! He basically didn't sleep at all during the day for his first four and a half months - now I'm worried that he sleeps TOO much because he'll sleep for hours at a time. He's asleep more than he's awake. Total 180.
ETA: Most nights DS's asleep at the same time - 7 pm or thereabouts. On nights when he's just not going down, I'll give him a long bath with me. It doesn't hype him up but it does keep him entertained enough that he doesn't work himself up into a fuss either. He usually falls asleep after. I'd say if your DC is fussing for a LONG time before falling asleep, and there's not anything actually wrong with him, don't force it. If he's not asleep within 5-10 minutes, just distract him with a quiet activity for a little while, then try again.
ETA: Most nights DS's asleep at the same time - 7 pm or thereabouts. On nights when he's just not going down, I'll give him a long bath with me. It doesn't hype him up but it does keep him entertained enough that he doesn't work himself up into a fuss either. He usually falls asleep after. I'd say if your DC is fussing for a LONG time before falling asleep, and there's not anything actually wrong with him, don't force it. If he's not asleep within 5-10 minutes, just distract him with a quiet activity for a little while, then try again.