Could it be photons are not so easy to control as electrons when creating an on/off switch?

Yes, exactly. This is because photons are electrically neutral as a whole, they carry no net electric charge, positive or negative. Thus they are not influenced by things like "voltage".

So to modulate the behavior of photons as a switch like in a transistor, also using other photons to flip the switch, would require a mechanism that works on completely different principles than an electric PNP or NPN transistor. Plus photons don't interact with other photons, they pass right through each other. This makes it very difficult for them to affect each other. Basically there would have to be some matter (probably an electron or proton) that absorbs the photon's energy that acts as a switch somehow because the photons cannot directly affect each other. Electrons hit each other and repel each other because of their charge, but photons flow through each other without affecting each other. So there is no way to create photon "pressure" aka voltage, so the design of the transistor has to be completely and totally different.

Maybe a fluorescent material might work...

It's easy to make an OR logic circuit with photons (you just combine the streams), but no one has figured out how to make an AND circuit with just photons, or any other type of logic gate. Even the simple NOT gate hasn't been figured out... how do you make a physical thing that when it detects light, refuses to send light. But when it sees no light, it lets light through from a light source. How do you do that with zero electronics, just from physical materials? That's the billion dollar question.

And also what frequency are the photons used, and why? I assume the material used would have some property that's best exploited at a certain frequency...

These are the main unanswered questions with inventing a photon transistor. If you can figure out how to make a photon AND gate, you will probably be rich for the rest of your life.