Red junior balls should not be used since they are oversized. Kids can use these balls, as they're made for easier hitting. It is impossible for them to fit into the chamber of a ball machine, resulting in the machine jamming.
A pressure-free ball is a solution. Unlike other balls, these do not use pressurized air to create their shape. The balls will continue to bounce and have the same shape for many years to come. A pressureless ball should last between one and two years on average (depending on surface and usage).