When I use Phantom of Chaos on The Winged Dragon of Ra Phoenix Mode, sometimes I can special summon The Winged Dragon of Ra Sphere Mode, and sometimes I can't. Which should it be? I have a feeling the reason I don't special summon is probably some card that says can't special summon this turn, but it still happens even after I took out Pot of Duality.

I can't seem to find anything on this. The closest thing I found  was Phantom of Chaos and the regular The Winged Dragon of Ra:

Q: If a Phantom of Chaos that has its card name treated as The Winged Dragon of Ra is sent to the Graveyard, does the effect of a The Winged Dragon of Ra – Phoenix Mode in the Graveyard activate?
A: In the proposed scenario, since this is not treated as The Winged Dragon of Ra being sent from the field to the Graveyard, the effect of The Winged Dragon of Ra – Phoenix Mode does not activate.