Hi - What do you mean not all the foam goes in? I have been using predfoam on and off for years now. Some nights, after I squirt it in, some does come back out - especially if i'm flaring badly and my ass refuses to have any of it! But, usually most of it stays in.
How much comes out? If its not a big amount, i wouldn't worry about it.
PS - Make sure you lie on your front for around 5 mins after doing it.