If you are attempting to find min time for maxim black you will need to be stopped down to f8 or f 11 and use a blank negative or the rebate of processed film. Do a step wedge across the paper at 3 sec intervals. Process for 90 secs.

A test without a negative doesn't mean much. Leave the fstop constant and change the time, not the other way around. i.e. don't start wide open and stop down.

8 secs is too short.

Did you make any other prints? What is the light source? Cold light heads tend to use more time than condensor heads.