Should agents use form P87?

Please help!

In July last year I set up a company which helps mainly pilots and cabin crew claim back overpaid tax.

Each client signs a document or indicates electronically that they are happy for me to act on their behalf with HMRC.

I have submitted P87 forms for each client and up until now most of them have been approved, lots after a bit of wrangling with HMRC.

Today I get a letter rejecting 52 claims saying the client has not signed the paperwork! Up until now it was fine and now it isn't? I have authority from the client to act on their behalf with HMRC. I'm obviously doing something wrong and I'm not sure what it is. 

Do I need to send P87 forms for each client? At the moment I have been sending one for each tax year but maybe I have been creating more work for myself, and wasting paper in the process!

Thank you for your help.

Nicola