If your broom is getting on or hasn't been pruned well in the past there may be stems with no leaves at the bottom and leaves at the top. Prune to an inch or two above the leafless stems. If you prune into old leafless stems they do not regrow.

Hard pruning of brooms like this each year stops them getting leggy and leafless at the bottom of stems an encourages bushtness

Clip over with shears, taking off the spent flowers and about an inch of growth.