Custom masks?

[FanofSMBX]

If you look in SMW Blocks section of ultimate graphics pack, "Dark Blue Cave Invasion 2 Plus Hammer Block", you see a rounded stone over hammer break block. Even though the default one doesn't have a mask, once I put it in my root graphics folder like the tutorial says, the custom mask works!
But I can't get the brick block or the SMB2 door to have a custom mask like this - why doesn't it work with those?

[bossedit8]

If a graphic that does not have a Mask in it, you can not add a specific Mask on a graphic that does not have a mask before.

[FanofSMBX]

I could on the grey hammer block, but not anything else. Why is this?

[FallingSnow]

If you are referring to block-457, I guess you found an exception. What bossedit said applies normally to any block that does not have a mask in the graphics folder. You can't give a mask to a block that doesn't have one normally.