I would ask you first: what cut of meat are you using? Also, always bear in mind that tenderness -or at least the tendency for tenderness in a cut of beef or pork- usually depends on two things: what portion of the animal it was taken from and the age of the animal. Cuts taken from the portion of the animal with least fat is generally tougher than those taken from fattier areas. And the meat of older animals is almost always tougher than animals in their prime or younger.
Otherwise, overuse of salt has a detrimental effect on the tenderness of meat. As strange as it may seem, overuse of meat tenderizer -which incorporates a great deal of sodium- can harden the tissues, even if you baste well or roast in liquid. If the cut and age of your roast is suitable, I would suggest you steer away from meat tenderizer seasonings altogether. Allow the roast to cook in its own juices, and season it minimally at most. And never let the roast go dry. If it appears to be sizzling in the crock pot, pour in some natural broth or water with a little olive oil.