It is odd, as Bio is normally quite e flexible gel, and doesn't crack. I can only assume you are using S-gel as your main builder layer...does the same happen with colour overlays?
Couple of points to check. Make sure your sidewalls are nice and thin....too thick an application here would mean the nail would lose flexibility, and make sure there is no soft tissue contact.
Also, have you built a good apex? If its too thin here, it could flex too much and stress fractures appear?
I presume you are using all Bio prep and are happy with it?