Thin FP4+ Negative

Processed some large format (4x5) Ilford FP4 using a 1+4 dilution of Ilford DD-X. Negatives came out extremely thin despite ensuring the chemicals were mixed 100% correctly. Here were the steps taken (everything at exactly 20C):

-Prewash for 2:00
-Developer (100ml DD-X + 400ml water) for 10:00
-Stop bath
-Ilford Rapid Fixer for 5:30
-Wash for 10:00
-Photo Flo for 1:00

Gross underexposure? There are a few areas of the film that are very thick (i.e. appear black on the negative) close to where the dark slide inserts into the film holder. I can post scans tomorrow but wanted to put this here just in case.

FWIW the Acros that was processed with this turned out great.

Thank you for the response. This was processed in a temperature controlled Jobo CPA2 processor and everything including the water baths were at 20C.

Do you remember the exposure you used (f stop - shutter speed)? Indoors, outdoor...?

Film loaded in the slide the wrong way so shot through the back instead? We've all done it!