The only time I've seen something like that was when the number of objects in the Oracle database exceede the capability of the report designer to display them. In that case - stored procedures are generally the first ones to not be displayed.
Do you have a sql server you can test with - or perhaps verify how many objects you have in your database? I believe the default is 50k objects as the limit.
Also, what user account are you using to log onto Oracle? I assume it is one that has sufficient rights to see the stored procedures. There is a kbase about using the wrong account here:
https://support.wdf.sap.corp/sap/support/notes/1980814
I assume that you are given that the stored procedure show up in the data explorer - but it doesn't hurt to check.
Shawn