R.M.
C.-
I'm a former Nanny and current Parent Coach. When parents have this trouble, I often ask them if they noticed a sleep pattern change earlier than the trouble started, say 6 weeks ago. Depending on the answer, we move forwards, so first think about that.
Secondly, I suggest to parents to lay their child down for sleep 30-45 minutes earlier than they have been. After years working with nappers, I've found they have a "window" for sleep, and if they pass that window we cannot get them to sleep to save our lives. Since an infant's sleep cycle is constantly changing due to age and growth spurts, a child who laid down at 1 pm may now need to sleep at 12:30 because they've used up all their energy. But when the 1 pm nap starts, they are so over tired, they cannot sleep.
If there isn't something in the prior 6 weeks which explains the issue, try an earlier bedtime.
Good Luck-
R. Magby