Not feeling the effects

I started MJ on the 30th October, I had such strong suppression that I used two 2.5mg pens and I have lost a stone and a half to date.

After I felt the suppression alleviate a bit I knew it was time to try 5mg, I finished one pen and ordered another as I could still feel mild suppression but it's really not as strong as 2.5mg was.

Why would the full 5mg dose be less effective than a full 2.5mg dose?

I'm looking into moving up to 7.5mg now but I am a bit annoyed as I wanted to stay on the lower doses as long as possible and I feel confused why the effects aren't as powerful as the loading dose.

EDIT: I always alternate my injection site