Rockwool stacking rooting issues.

Hi fellas how are you?

I've got an issues and I can't seem to figure out what's happening. I've got a veg room with 4x4x4 Rockwool cubes(Ebb and Flow, res. change every week, Jacks), the plants are pretty healthy, roots poking out of the bottom, but when I put on the top of the 6x6x4 in the flower room some will root in to the new cubes some will refuse.

What cold be the problem? Something is off in the Veg?

I'm vegging under the HLG 600 B SPEC running 50%, flower is to feed with FloraFlex 4" two lines.

Any insight would be very much appreciated.

I cant help with the current problem, but in the future if you go with a 6x6x6 rockwool block you shouldnt need to stack.
Read a test using 4x, 6x and 8x blocks and my take-away was the 4x too small, 8x too big, 6x plenty of room.
Been using the 6x with no problems for years.

Hey...make sure the EC is less in the 6x6 than the 4x4...the roots will want the lighter nute strength which will promote the travel to the next block.. Also make sure that the roots have really taken up the 4x4 well before going to the 6x6. If there is still plenty of room for the roots to populate the 4x4...they roots may not need or want to go to the next block a aggressively. Make sure that you buffer the blocks with a 24 hour soak at 4-4.5ph. I do mine twice. (Grodan) First soak at 4.0 then the second at 4.5 with nutes. I usually end up right 5.5-5.6...hope this helped