Active end range of motion limitation (full grip weakness) after hand flexor tendon repair

Hello

I am faced with this case where after successful repair surgery of hand flexor and extensor injury and successful rehabilitation programme resulted in full passive range of motion, excellent muscle strength, good scar tissue no adhesion or pain.
However the patient (who has full passive grip ROM) is unable to perform full active grip.
MP active ROM is normal the problem seems to be in the active end range of motion of PIP and DIP.
After searching the net i came to the concept of post-surgery tendon elongation that results in slacking of the tendon at the end ROM.

I would like to know if somebody can provide me with information about the above case and how to treat to achieve powerful full grip.
if elongation is the cause is there another option beside re-surgery.

thanks