Heathers can get quite woody if not pruned. As soon as they have finished flowering, you can give them a haircut, cutting just above the old wood.
If they are getting really old and woody, it may be better to propogate new plants and discard the old ones. To do this, dig up the plant and replant it deeper, filling the hole with soil so only the tops of the shoots are above soil level. By the end of the year, each shoot should have made its own roots and can be cut off the old plant and replanted