Did you follow The_Z's tutorial? You need to edit the PATH.TXT to the game folder you want to use as base, and use fake_np to fakesign VHBL.ISO with that game's information.
Also, if you didn't after running the exploit remember to rebuild your database in safe mode (From power off, hold Power+Home+R). That way the Vita can refresh the information in each bubble.
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Always follow *all* steps of the friendly tutorial. They aren't there just as 'guidelines.'