Up-to-date schedule for the DLA-PIP process

If it's helpful, both my adult sons were on indefinite awards of DLA. Both live at home still. Both have the same surname, in case you thought it might be alphabetical

One son got his invitation early this year, and has now got PIP.
My other son has not yet received a letter saying he needs to apply for PIP.

As Gordon says, it seems to be completely random.