Courgette pollination

I have 3 courgette plants in my greenhouse & have been hand pollinating my courgettes by rubbing the male flower pollen on the females. I'm still get courgettes that aren't being pollinated properly - each plant only seems to have one courgette growing properly at a time. For some pollinations I've had to save male flowers for up to three days as there aren't any fresh ones - I've had them in the fridge, is this killing the pollen and causing my issues? Or do my plants just only focus on one fruit at a time? I'm growing the greenbush cultivar in large grow bags but am confident watering & feeding isn't the issue.