why does mask ?d work but mask ?a not?

[klong]

Hi all, I do not understand the ?a correctly I guess.
Because when I type:

hashcat.exe -a3 -m110 -O --hex-salt 04d661b3e8f4e1ce8fd8e64188314c8cb2463953:d7e72797 ?a?a?a?a?a

it will not recover "Pas33", but when I type:

hashcat.exe -a3 -m110 -O --hex-salt 04d661b3e8f4e1ce8fd8e64188314c8cb2463953:d7e72797 ?a?a?a?d?a
or:
hashcat.exe -a3 -m110 -O --hex-salt 04d661b3e8f4e1ce8fd8e64188314c8cb2463953:d7e72797 ?u?l?l?d?d

it recovers "Pas33"


What did I do wrong (using hashcat-6.1.1 in win10-64)?
Thanks in advance

Klong