also make sure that your brush tails are not getting caught up anywhere on the endbell as this also can cause a hung brush,

From what you are describing it sounds like either there is not enough brush material left to allow the brush to run firmly against the comm or something is catching the brush tail and again not allowing the brush to contact.

I have also even seen it happen when the brush hood where the brush goes in is knocked or bent from hitting something which also causes the brush to stick and not move forwards in the hood.