Office collision: 7 or more characters?

[Mem5]

Hello !

I am trying to collide an Office hash : $oldoffice$0*aa*bb*cc. I use :

9710 | MS Office <= 2003 $0|$1, MD5 + RC4, collider #1

hashcat64.exe -w 3 -m 9710 office.hash  -a 3 ?b?b?b?b?b  -o tmp_office.txt

This command give me this RC4 key :

$oldoffice$0*aa*bb*cc:2c2c2c2c2c

Then, I use 9720 MS Office <= 2003 MD5 + RC4, collision-mode #2

hashcat64.exe -w 3 -m 9720 tmp_office.txt -a 3  ?a?a?a?a?a?a?a

After 7 hours, exhausted, but no password found.

Does it mean my charset (seven times ?a) is not big enough ?
I though this mode (9720) would find a collision with only 7 characters.
8 times ?a is huge and long.

Thanks for your help.