The polymeric sand "grout" between my slate patio pavers is loosening up and coming out. I suspect it was improperly laid by the contractor. I am going to redo the polymeric sand treatment and I am thinking about using a patio sealer such as Thompson's Patio and Block Paving Seal to brighten up the stone and protect the sand from breaking up. Does this sound like a good idea or is there a problem to doing this that the presence polymeric sand might present?
It's normal for it to loosen a bit the first season or two as things finish settling. If you're in a freeze/thaw cycle climate that can also cause that.
You should be able to apply your sealer over the sanded joints, though it may not prevent this issue from happening again. Patios take maintenance. I had to re-apply a new layer of sand every few years to fill in the gaps here and there.