Mine have bred anywhere from 24° – 28°C, but I have heard of them breeding at 22°. Your water may be a little cold, but perhaps you read up on them and know that 19° – 20° is fine.
Other than possibly raising your water temperature slightly, I think you may just need to give them a little more time to get used to their surroundings — it sounds like you have everything set up well.
Their eggs are very small. If you pull the adults out in a couple of weeks, you may find that you have plenty of fry a few days later.