Posted: 23/03/2015 at 12:18
It sounds to me as if the shrubs are in need of re-potting. The soil has become acidic, compacted and stale. Remove the shrubs from the pots and repot either in John Innes No 3 or ericaceous compost if they are ericaceous plants. Make sure there is plenty of drainage in the pots and add extra grit if needed.
When repotting first give them a good soaking then tease out some of the roots to make sure they're not rootbound and gently remove the moss and soil from the top - put them into slightly larger containers so they will have new compost at the bottom, around the sides and on top.
Finish off the top with gravel or grit.