Vacuum Suction

I've got a strange problem I absolutely cannot explain.

Cutting some aluminium composite, completely non porous. Spoil board is 12mm LDF.

I cut out a letter 'T' roughly 300mm x 200mm, if I place this directly onto the spoil board with all zones open I get a decent hold down force.

However, if I take the sheet from which the T was cut (using a 6mm end mill) and lay that onto the spoil board and place the T in the cut out section I get absolutely no suction hold down what so ever.

How can this be? Surely with the sheet on the spoil board effectively acting as a cover board sealing the spoil board the hold down should be increased in the cut out area rather than decreased?

Any suggestions? Edges are sealed of spoil board and the 'T' is being placed in the same area both with and without the sheet.