Do you have a wind down routine before naptime?
My 5 month old used to nap 30-45 mins, and he would never go down for a nap whenever I wanted him too. Then I put him on EASY, started looking for his sleep window, when he started to look tired I would take him to his room and spend 10-15 minutes getting him ready for his nap: diaper change if needed, swaddle, dim lights, sit with him for a few minutes. After doing that he started going to sleep right away. Then when he woke after 30-45 mins, I did PU/PD until he went back to sleep and he would sleep out the rest of his time. It was hard the first few days, he would protest PU/PD and it would take up to an hour, but I stuck with it and now he will often sleep the full 1-1/2 to 2 hours straight, and if he does wake it takes little time to get him to go back to sleep. We're not there yet but we're getting there...Saturday he slept two hours straight in the morning and 1-1/2 hours in the afternoon without waking, first day I had two successful naps. I was ecstatic - progress!!!
Also, what sorts of activities does he do before naptime? Maybe he is too wound up to sleep. I used to let my LO play in his jolly jumper hoping to tire him out then try to put him down right away. Turns out he was too worked up to sleep. Now I try to keep the activity time low key for about 30 mins before naptime and that really seems to help.
My son does the mantra cry too, so when he does that I just try to soothe him in his crib. I use shush pat or hum, sometimes just put my hand on him and it seems to be working. I'm thinking actually of just leaving him be if he is doing the mantra cry and seeing if he settles on his own...
He used to do the same thing, wake up after 30-45 mins and look like he was happy and rearing to go, so I would get him up then 30 mins later he would be cranky. Now I make him finish his full nap, using PU/PD if I have to. It's really been helping us.
Good luck!