How can I stop my 11 year female Burmese from spraying?

We have two female Burmese Cats both 11 years old. Poppy has always been very timid and nervous but over the past 6 months she has started spraying on a regular basis. This is now becoming quite a problem as she does not stick to one place and cleaning up is a difficult chore especially on soft furnishings.We have tried giving her Serene which is a natural seratonin based treatment which helped for a while but two months down she is worse than ever. Any help would be appreciated