Hi stomperone, welcome to WAB
I doubt if the presence of a few magpies would stop other birds exploiting a food source. It may just be down to the time of year, wild food is plentiful at the end of summer, so there's no pressure on birds to find alternative supplies. How long have your feeders been in place? If they're fairly new, the birds may simply not have found them yet.
Lack of shrubs may be a contributory factor - a lot of birds don't like to feel exposed when they're feeding, so feeders should placed close to good cover. Do you have a water feature? A supply of drinking/bathing water helps attract birds into a garden.
HTH
T2