I can spend sometimes half an hour or more settling my DD for a nap and she'll maybe only sleep for 20/30 minutes.
I find it infuriating as she is so obviously tired but fights it! She'll shown sign of being wide-eyed and awake which totally throws me!!! Then if she does finally fall asleep she'll wake up 20/30 minutes later.
My DH says I should only try for a short while and if she clearly won't sleep don't bother! Take her out, put he on her play may or bouncy chair and leave her.
I sort of agree as it is an utter waste of time but then why show all the tired signs beforehand?????