I knew I had read something about these worms in various old books, and actually found one which explains clearly that there are an unknown but massive variety of these worms — also called eel worms.
Two of them are responsible for the bulging bumps on roots of tomatoes and potatoes and the plants demise. There are even varieties who live in the sea.
I still cannot tie down why the nematodes purchased by gardeners die out after a short time. Unless it is that only one sex of nematodes are delivered, thus reducing the chance of increase in numbers to a far smaller amount. From what I gather they are sexually reproducive rather than anything else.
Since they can be quite expensive, I thought it wise to try to find out why they dont just multiply and multiply. I suppose there is a natural predator, but quite what that is when these little guys can eat snails and slugs, I dont know.
Just FYI
Julie