Photoshop keeps deselecting the layer I'm working on

I'm moving and duplicating parts of an illustration around on a single layer in P'shop. It's the only layer other than Background. When I select something with the marquee tool, then hit M to get the move tool, an error message says I cannot use the move tool because no layer is selected. So I select the layer from the layer panel. The next time I use the marquee, the layer I was working on, is somehow de-selected again. This keeps happening. Why can’t I stay on the layer I”m working on?