Screed surface dusty and loose

I have now been laying a 65 mm floor screed. I mixed it very dry as people advice. I also added fibre reinforcing. But in places the surface is loose after 2-3 days of laying. You can erode it with the finger until 3-4mm deep before reaching a hard surface. This happens especially in day joints and where I flattened it with a straight edge that had a sharp edge, before realising that rounding off the edge was better. The sharp edge was dragging the fibres and tiny pebbles in the sand when passing it over the surface.

I have checked the core and it is quite firm.

The question is what do I do about it now? I am going to lay tiles on top of it and it has underfloor heating below.

Another question. I am using polythene sheets to control the drying. For how long should I leave the the sheets in the present weather in the Midlands?

did you 'compress' the surface with a floor trowel?

I compressed it as I laid it, but on those areas where the problem is, I dragged the straight edge over the surface and forgot to compress it again

first you screed it level with a straightedge.

then you rub up the surface with a polyfloat, filling in where necessary.

next you trowel finish the surface with a floor trowel. this final application tightens up the finish on the floor surface. similar to skimming a wall, it compresses the grain and makes for a tougher surface.