I bought some Slime to fix a puncture today and it worked brilliantly on my front tyre, the slime went down the tube and into the inner tube without any problem.

When I tried to do the rear tyre, the slime was only filling up the valve and not actually going inside the inner tube. I can inflate it without any trouble so doesn't seem to be a blockage.

Does anyone know why the slime won't go in? It's the same type of tyre as the front wheel.

Thanks for any help!
Mark