IME the taproot (and then the stem) grows very quickly after the seed first cracks open, so i'm surprised you say when you opened the cubes after a few days "Some of the seeds had not sprouted much at all" Are your temps ok? And are the taproots properly in contact with the rockwool and able to take up water? (did you notice if the taproot was moist and soft when you opened the cubes)

Other than that like metal says a humidity dome will help along with a little patience.

Good luck!