Getting toddler to sleep through night

Does anyone have any advice on how to get a 19 month old to sleep through the night ?

She’s still in a crib. Takes one nap a day from 1-2 hours. If she doesn’t nap she’s extremely cranky.

She was sleeping through the night once in a while but lately has only been sleeping for about 3 hours at a time. She will wake up and not go back to sleep for a while. Or will sleep while being rocked but wake up as soon as she is out back in her crib.

Sometimes she will stay asleep when put in crib but wake after 20-30 mins.

Is this a regression? Teething ? A phase ? Separation anxiety? I’m about to have another baby and her sleep habits are going to ruin us all !

Help!