Different behaviors when creating defect WIs

In Web environment, there are two options in order to create a new Work Item:
Scenario 1
1-Click the "Create Work Item" link, on the left frame, that opens the "Welcome to Work Items" right frame.
2-In the right frame, click the "Create Work Item" link
3-Select the Work Item type that you want to create, from the 'Type' drop-down list.
... and so on.
Scenario 2
1-Click the little arrow next to the "Create Work Item" link, on the left frame, that displays the list of available Work Items.
2-Select the Work Item type that you want to create from the list.
... and so on.

Our problem
We have a customized Defect WI.
If you choose to create the Defect WI like Scenario 1 describes, then custom attributes are disable (only the enumerations).
But, everything is fine if you follow the steps that Scenario 2 describes.

Any ideas? Are we forgetting something?
Thanks in advance!
Silvana