[slunt]

Don't know why you are using lemon juice, but it's definitely not necessary for cloning.
Air pockets are rare also, not much of a worry usually.
Your temps are too high. 25/26 C is the aiming point.
The plants transpire moisture when its hot and can't take any in because roots are not developed.
Rock wool needs to be damp, not wet. The plant will take a long time to grow roots to seek moisture if the stem is wet or submerged.
What kind of light are they getting? CFL bulbs are good for this or any cool Floro tube will do at this stage. 24 hours on.