Have you kept it trimmed after flowering, most hebes respond well to pruning however if they are older specimins then it can be laboriously slow if at all, could you post a picture ?, any small shoots near to the base that you could prune to, with certain hebes over the past, i have adopted a method called mounding, where you earth them up, its a good propagation technique, or lift the hebe, dig the hole bigger and by doing so you cover all the straggly growth leaving just the healthy foliage, the effect is instant looks like you have planted a new Hebe, then trim after flowering to keep the plant looking more compact.

i too was going to suggest pruning. I've heard of mounding as a solution but never done it.