Mask attack issues

Hi All,

I recently faced an issue with mask attack trying to crack a bunch of NTLM passwords. 

The command I used which did not work is: hashcat -a 3 -m 1000 hash.txt -i --increment-min=8 --increment-max=15 ?a?a?a?a?a?a?a?a?a?a?a?a?a?a?a

It tried all 8 character and did not crack a single password in the hash.txt list (There were a lot of 8 character passwords)

I tried a command without trying the mask: hashcat -a 3 -m 1000 hash.txt -i --increment-min=8 --increment-max=15

The above command cracked a bunch of passwords. (I know it takes a default mask and it is a smaller subset)

Not sure why the first command did not yield any passwords. Can somebody suggest or help me with this issue.