In some of Kstarr’s videos, he always states that the pain should grow less at the end of the session, not more. 

Also I think pressure should be increased gradually as you detect change in the tissue. You should add pressure gradually as the tissue adapts and relaxes. Some of the reason for non-change is because your body doesn’t accept the pressure.  Thus, your muscles stay tense in reaction to the pressure and resulting soreness/pain from it, and that is not what you want.  So you may need to apply lighter pressure until your body and brain accept the pressure, and finally lets the tissue relax into it.  Applying more pressure when the muscle has not relaxed will increase the chance that you cause trauma to the muscle.  That is why we accelerate change by tighten-then-relax.  Smashing into tight muscle without some neurological change agent will ultimately bruise it.

I have found that sometimes, no matter what i do, i cannot relax into the pressure. So i just stop and go at it either later in the day or the next day.  

It also can take time to get used to “smashing”.  Over time you develop the ability to “accept” smashing and the positive results it brings. When you start out, or if you have spent a long time away from it, it can take a while to adapt.  So it may mean only 30 seconds for the first few sessions, then build up to many minutes.

Different parts of your body may have different reactions to smashing and the type of tool used for smashing. For example, my right high hamstring gets very tight sometimes. But I cannot use a hard lacrosse ball or supernova on it; it reacts badly and it gets more sore after. HOWEVER if i use a softer Alpha ball, i can get the tissues to release. Completely different situation on my left hamstring where i can use hard lacrosse balls or supernova on it no problem.

I would let the tissues recover before smashing again, then either go at it for less time, less pressure, or use a softer tool until you build up your tolerance – and the tissues show more positive change.