1. Is the ph of the water close to neutral? If not investigate possible cause of alkalinity.
2. Is the pool free of rotting vegetation ( I know this is the hard bit ...gardening with wellies ).
3. Check that nutrients are not entering the pool via runoff from the surrounding area.
4. Are domestic animals and wildlife ( ducks, geese etc) using the pool excessively?
5. Are the plants within the pool suited to their environment? Have their numbers been drastically reduced by what has been a hard winter?
6. Are the plants in your pool good filter feeders?
7. Bubbles floating on the pools surface can indicate excessive nutrient levels.