Backdated PIP?

My husband was asked to change from dla to pip in may 17. He was refused pip as he could not go to an interview due to mental health. His dla was also stopped. I appealed for him and the court said that dla had to be reinstated until pip gave him a fair interview. Pip then reassed his claim and decided to award him high rate on both elements without an interview. He was only getting low rate mobility on dla.  I telephoned pip to ask if it would be backdated to the original claim that said it wouldn’t. There is quite a difference between the amount of money from dla and pip and this has been going on for almost 18 months. Is this correct . If it is it doesn’t seem fair that he has lost out for so long due to pip not sorting it out properly. Any help would be gratefully received