Umm... ya.. rockwool is has a high ph... in order to use it you must adjust your water's ph down to 4-5 and let your cubes float around in it for 24 hours prior to propagating.... also fyi you shouldnt keep your seeds under direct light... and keeping the soils temperature above air temperature by using a seed/heat mat will definately help too.
if youre using hydro after propagation i assume you already have a pH test kit... so.. use your pH down to get your water between 4-5.5ish (lemon juice will drop it too if u dont have pH down). Put your rockwool cubes in the bowl of water overnight... this does 2 things.. it dechlorinates your water and lets the rockwool fully absorb... Rockwool maintains 18-20% of air so it takes awhile to fully absorb all your pH adjusted water.
Also you shouldnt germinate in rockwool... you should germinate a taproot in papertowels ...then when u get a taproot stick it in your rockwool. Rockwool is notorious for pushing out seeds when germinating.. it can be done its just annoying to most people.
Then, place your seeds on your heatmat in a bright place but not under direct light....