Originally Posted by
Mickmeister
It doesn't seem like a puncture...if I try to inflate it I can hear air leaking out of the valve.
Did you investigate this "leaking out of the valve" thing?
If the air is truly coming out of the valve, a likely suspect is the valve core:
If the valve core is loose, try tightening it.
If the valve core is replaceable, try replacing it.
If none of the above, then send the tube to your nearest landfill.