Hi OChydro,
Rockwool isn't exactly the best solution for lettuce in NFT, not for germination and for nursing neither. Actually roots tend to spread laterally in rockwool. I recommend special Lettuce cups, filled with a mix of 1 part Vermiculite and 3 parts of Perlite (both fine grade).
Also, (even if using rockwool) you can't transplant until the roots have grown long enough to reach the bottom of the gully! Just wait until they are at that stage, anything else is pointless (no offense).
PS: use adequate
nutrient for lettuce and the very adequate concentration to grow your seedlings, (starting with PH corrected, low ppm, RO or even distilled water for germination and beyond) otherwise you have delayed root growth and loose "vital" time in the ongoing operation.
If you need pics of the lettuce cups, nutrient requirements or other hints, just call in again...
Cheers,
Luches