Black masonry paint looks great wet, dull when dry

We have concrete window sills which are painted black with regular masonry paint.

When I wash them they look great, but when they dry, they're dull and dirty looking.

Is there anything we can apply over the masonry paint to give a glossier finish or is there anything we can add to the paint in an additional coat?

Thanks.

I must say that I prefer the matt black on my garden gate than gloss black. However it is difficult to get the right matt black colour. Have you considered just painting them in a standard black gloss paint?
I used to paint my window sills in a previous house using a standard matt white paint and it looked fine. Every few years I would freshen them up with a new coat.