A few times now, I have found that Red-Eyes Retro Dragon and Return of the Red-Eyes effects do not activate with just Red-Eyes Slash Dragon on the field. Am I missing something? Has anyone else experienced this?
1- When you use "Red-Eyes Fusion" in order to summon Slash, you cannot Normal or Special Summon in that turn (but you can Set, although the game won't tell you that).
2- I assume you are talking about the opponent destroying Slash at the turn it was summoned with Fusion with some effect while Retro is in your hand. In this case, you cannot activate its effect due to Fusion's restriction.
Like the guy said, Red-Eyes Fusion's card text says "you cannot Normal or Special Summon other monsters the turn you activate this card".
Both cards effect cannot be used during the same turn you used Red Eyes Fusion.