Crop page based on trim box not working

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Hi everyone,
I have a 1-page file created in Illustrator. I exported this file to pdf and have applied trim marks. Back in Acrobat Pro, I go into the crop pages dialogue box and choose "trimbox". When I do this, 11.642 mm is automatically inserted into the margin fields for all 4 sides. When I click OK, I expected the file to be trimmed to A4 (210 x 297) but no change occurred in the file.
I wondered if anyone knows why this has happened or what I'm doing wrong?
Appreciate any help.
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Replay

you need to tell Acrobat the measurement:
click on trim: copy the measurement in the margin control
click crop, check the constrain proportions box and paste trim measurement
click ok
G