Nap advice for 21 month old.

Hello! My toddler is 21 months old and we are having a hard time finding the ideal nap time. She used be to a great naper before changing to 1 nap. I currently put her down around 12 when she wakes up around 6:30 or 7. She will nap for about an hour and 15 minutes then wake up crying. She still seems so tired but won't fall back to sleep. She goes to bed around 7:30 and we start the process again. She never used to wake up crying so that tells me she isn't napping long enough. Any advice?