First time running a CNC so still learning but doing my best

8x4 bed, 11hp vacuum pump 18mm standard MDF.

Vacuum hold down was decent but not great on parts smaller than 300mm. I followed advise, brought a flycutter from my tool supplier and skimmed 0.5mm of the MDF, flipped over and sealed edges. The hold down was improved so I thought I'd go ahead a skim the second side also.

Huge mistake, it's like the vacuum pump isn't even on anymore. I tried again with an 18mm mdf offcut and the vacuum holds as it did originally, skimmed one face and again improved, skimmed second face and total loss of vacuum.

Anyone know what I did wrong? I'm attempted to hold down a full 8x4 sheet of ali comp, completely non porous so I shouldn't be loosing vacuum to unsealed spoil board portions.

Really got me scratching my head, the only negative I can think is when we skimmed we was left woth a slightly 'furry' surface, I did play around with feed and spindle speeds but results always look 'furry'.

For reference I am using a 25mm skimming tool, 18000rpm amd 90 ipm feed rate.

Have also ran pump in reverse, checked for leaks (small one around filter, sealed that while cleaning filter out)