Could be a few things. First a question, "Are their necks pulled out?". 1) Reed strength. Are they on 3's or 3 1/2's? If the Reed is too soft it can cause flattened pitch in the low register. A stiffer reed can aid pitch, only coupled with a 💪 embouchure and enough air pressure.
2) Reed age. If the reed is old it could be softer than it was originally. Soft reeds go flat.
Sometimes the Reed can be brand new but, gets water logged. You can see this as a Stark darkening around the tip with the rear of the Reed being very dry.
If the Reed is old change it out. If they can't handle a 3 1/2 have them push their reed slightly above the tip of the mouthpiece. This will stiffen it a bit. Then they must squeeze the embouchure corners and voice the notes up by raising the back of their tongue and pushing fast air. If they squeek it is because the back of their tongue is too high. I hope this helps.
: Why are my students bass clarinets low E so
: From open G down to low F sound great and in
: Thanks for the suggestions