If a student has gone through the entire program and has not shown any progress, it is very likely there is some kind of medical issue that can hopefully be identified and addressed.
If a student is in third grade or higher has not gone through the program in grades K-2 and is struggling, they can go through most elements of the K-2 program and work to catch up. The older the student the more difficult it usually is to catch up. In most cases they have learned to compensate and have established a fairly strong mindset of failure.
The first step is to explain the model to these students and persuade them that this struggle is not their fault. Together we can find the likely underlying cause(s) of their struggle and work to address most any barriers.