Hi Fred,
By Arum Lillies I assume you mean Zantedeschia aethiopica? In which case, yes, they do like a good rich soil and will take as much feed as you can give them, so, after 20 years, yours will have exhausted their supply of nutrients in their existing clump and the individual rhizomes will be competing with one another for what ever food and water there is.

They are pretty easy to divide, just lift the entire clump and you should be able to see the separate growing points. Then replant in some well worked soil/compost/well-rotted manure and they should be back to flowering strength in no time.

I agree with Ilex, but just wanted to add, watch out for slugs and snails who enjoy eating the nice fat fleshy buds as they pop out of the soil and that results also in no flowers, it happened to me last year after I had lifted and divided mine, so this year I was on the case early!