Sounds like the check valve is leaking back into the pump.
If it's like my cheap bottle jacks, there isn't much you can do to repair
them.. Just not worth the effort..
However before you toss it, try unscrewing the little control valve, all the
way out, or if it has a packing nut, remove it.. Becareful since there may
be a spring under a small ball valve inside.. Check that and see if there is
something in the seat..
If that doesn't help and you paid less than $10 for the jack, then just toss
it in the junk pile.. If you paid more that $25 you should try taking it
apart and repairing the pump or valve.. Without seeing the jack, I couldn't
tell you how to do that.. (mostly because I have never bothered to repair a
bottle jack other than the control valve)