I am fairly new to Minikegs with the party star tap but have been impressed - and I like nicely carbed (not flat) bitter.
I am topping up with CO2 after pouring every pint, rather than leaving the keg and topping up when ready to dispense the next pint. Just trying to keep the headspace nicely pressured and CO2'd all the time to protect the beer and keep as much CO2 in the ale as possible.
Also, have you popped a little vaseline on the plunge rod when tapping - and on the rubber washer around the top of the plunge rod before tightening into the party star tap before tapping a keg ? I think using a little vaseline on these parts must help reduce the chances of getting little leaks allowing the pressure to drop after tapping the keg during the time you are dispensing from it.
As an aside one Co2 bulb is lasting me 2 minikegs, pleased with that although they are the larger 16g CO2 bulbs.
Hope some of that helps and good luck with it.