As kids with active individualized education programs (IEPs) move between states, districts, or countries, their needs may or may not be fully recognized.
One district's tests might show that there is no need for an IEP, while the previous school had the same child in pull-out classes.
Services aren't always the same in each school or the therapists aren't of the same caliber. For kids with significant needs, this can threaten any positive progress they have made or limit the available duty stations because of Exceptional Family Member Program (EFMP) rules.
From the early referral stage to the final(ish) meeting, so many things could complicate the process. And that's if the child/family stays in one district long-term. Add in a move during the RTI or reevaluation process, and the whole thing could collapse.
Just getting an IEP is challenging.