Was the angler fishing bloodworm and joker? Salt in groundbait kills B & J? Groundbait irrespective has a higher feed content than Leam. Leam can be flavoured quite easily / successfully.

Leam, being heavier than groundbait and stiffer in balls makes more noise, and the initial attraction of balling it is the noise, not the smell, etc. So the fish will come to the noise, and find whatever food is in the leam. Groundbait is there usually to keep fish grazing, and they will happily eat it. You can then use the activity and particles in a g/b to attract fish from downstream and to stimulate them into feeding more confidently (will raisen uses alot of corriander in his river roach mixes, particles go downstream drawing fish up.

The main reason they stay and feed though, is grazing over the groundbaited area. So using leam is the same as using groundbait as a carrier for say joker, or casters, but has no food content to doesn;t fill them up.

Depending on the leam you use, is can also cloud more or less than groundbait. A normal lake damp leam with yellow dye, thown in regularly to put a yellow cloud through still water, can be deadly for skimmers for instance. A black leam can provide a low feed cloud for small roach in winter.