Things to consider:

1. Trees (and branches) move, these are living things and the seasons, weather, and time of day all effect them.

2. Cats, birds, squirrels and other rodents climb in and around trees.

3. Motion detectors should be mounted in as stable a location as possable, a wall is better than a branch.

4. Perhaps the motion detector does not need to face down, have you tried other locations / positions?