I have had the same problem of loss of highlights and it can occur if the sizing on the paper is not hardened quite enough; I think of the problem as a 'micro frilling' issue where the very thin highlight gelatine does not quite stick well enough to the support. The sizing is hard enough to stop much of the frilling issues, but not quite strong enough to hold onto the highlights. Try painting on a 5% solution of formalin onto a sheet to harden it some more and see if the highlights change.
I also have had the problem when the mating bath was too alkaline and the gelatine sizing on the paper gets a slightly 'slippy' feeling in the mating bath. A dash of citric acid solution (e.g. stop bath) can help in this case and I now slightly acidify the mating water routinely now as it always seems to have a positive benefit with image adhesion.