Probably because its in a pot - it takes a lot energy to flower, and even more to turn those flowers into berries, and in a pot, the plant probably can't afford the energy. Either that, or insufficient water during the flowering process and afterwards, or the flowers aren't being pollinated (unlikely if its outdoors).

Not a great loss though if yours is anything like mine - the berries are a lot smaller than the ones on the wild elder.

I would be inclined to put it in the garden to reach its full potential (can easily reach 12ft plus). I would imagine that feeding it with any fertiliser containing nitrogen would make plenty of leafy growth at the expense of flowers and berries. I would wait until spring and feed only with something high in potassium (tomato) fertiliser.

And in the ground it won't need any feeding...annual pruning will keep it the size you want.

Grows like billy-oh! Lovely though! Mine has improved the amount of berries since I put in a different one (Black Beauty)at the other end of the garden after reading that you need another one nearby. They both flower and berry profusely. Agree with Stera that the berries on Black Lace are small. I think winter is the time to prune if you want berries, but do check this. I want berries for the birds and have not pruned them in the last year.