Looking at the advice from the RSPB
- Boxes for tits, sparrows or starlings should be fixed two to four metres up a tree or a wall.
- Unless there are trees or buildings which shade the box during the day, face the box between north and east, thus avoiding strong sunlight and the wettest winds.
- Make sure that the birds have a clear flight path to the nest without any clutter directly in front of the entrance. Tilt the box forward slightly so that any driving rain will hit the roof and bounce clear.
- House sparrows and starlings will readily use nestboxes placed high up under the eaves. Since these birds nest in loose colonies, two or three can be sited spaced out on the same side of the house. Keep these away from areas where house martins normally nest.
- Two boxes close together may be occupied by the same species if they are at the edge of adjoining territories and there is plenty of natural food. While this readily happens in the countryside, it is rare in gardens, where you normally can only expect one nesting pair of any one species. The exceptions to this are house and tree sparrows and house martins, which are colonial nesters. By putting up different boxes, several species can be attracted.
There's no mention that branches or squirrels are an issue, but placement is important depending on what species you're looking to attract.
Perhaps the existing ones need cleaning out and repositioning so that they're not catching wind and rain and not too close together.
However do take note of the last point, birds are territorial, if you have lots of boxes within one territory it's likely that only one will be occupied.