It sounds really boring, but are the seals on your pump all sealing OK? Can you feel "suck" but nothing happens, or is it just not sucking very well?
If it is just after a feed then maybe there isn't very much there either - you should probably be at the stage by now where you pretty much make what bub needs and not much extra.
If you do need a stash then the most reliable way to do it is to add another "feed" to your day where you feed the pump rather than the baby - after a few days your boobs will get the idea and you will have the extra milk supply.