Why, and how can i get my 15mm back?!

1: There's a leak.

2: Get them serviced.

Next: My hair is getting longer, how can I shorten it? 🙄

I'll try to be a bit more civil.

Do you have more air in the neg than the pos chamber? I believe this can cause them to sit further down in their travel.

My new Rev is doing it too so I need to investigate this as well.

It quite possible there's an air leak from +ve to -ve air, which will drag them down. Service is the only cure for that

But

Try letting all your -ve air out, checking the +ve pressure, then resetting the -ve to no more than the same pressure as the +ve

That will give you your travel beck, but if they keep dropping, it's service time I'm afraid! 🙂

EDIT
Air Pikes seem very prone to this, I've heard of it on quite a few sets, mine included

Tempory fix – drop all the air out of the -ve chamber – they'll extend back to full travel, then refinflate. (worth topping up the +ve air as well as it's leaking out of there into the -ve chamber.

chakaping – Member
I'll try to be a bit more civil.

It's a dirty job, but someone's got to do it…

Good advice (barring Woppit obviously, and your wacky sense of humour) dropping then resorting -ve seems to have done the trick. This is a useful forum!