I have recently changed my mount board and I am noticing a dimple on my dry mounted prints. There is a very gentle, edge-long dimple parallel to all four edges of the print, less than 1 cm (1/4") from the edge. I can only see it when looking at the print at an angle at which I can see a light reflection along the edge, otherwise it is not visible at all.

The dimple is like a very shallow "valley" close to the edge of the print.

I am wondering if anyone has seen this before and if there was an easy solution, like higher temperature, more pressure, or the opposite. I use Unimount tissue (made in UK by Hotpress). I place the package under two sheets of larger 4-ply mount board and there is one such sheet under the print package. The press is a Bogen 560 (aka Technal).

Many thanks,
Rafal