Sax problem, any ideas on this?

I had a sax part recorded for one of my tracks elsewhere and sent to me as a wav. I added some reverb and sat it in the track it sounded great on speakers and in the cans. However, as soon as it was rendered in Reaper, it disappeared into the back of the mix. I have tried raising the volume of the sax track, taken the reverb down, changed the EQ, but nothing seems to solve the issue. Any ideas or tips would be most welcome.

If you render a stereo track to mono, and there's lots of out of phase reverb on the track, that can happen. A goniometer will show that. You've already changed the reverb, and I doubt if you're rendering to mono, but that's all I can think of, sorry.

You haven't sent the Sax audio to a bus that is being monitored but not rendered have you?

Yeah, that's my guess too. You forgot to route the track to the bus you intended so it didn't get rendered as intended. I do this to myself a bit too often too! :) Specifically you likely forgot to route the sax track itself to your output/mix bus (ie. left it routed to the Reaper master bus which you are monitoring). The reverb is routed to your output/mix bus as intended. You render your mix bus and the sax track (routed to the master which is only being monitored) is omitted from the mix. PS. Have you checked out the routing matrix view? Easy to spot missed sends and magnitudes quicker to route your mixing board. Also, you can uncheck the preference for automatically adding that master send on any new track. Since you will likely be routing any instrument tracks to a subgroup bus first anyway, this forces the issue as well as saves you the step of de-routing from the master.

If it does not disappear completely, only hides in the background, think of not changing the sax but the other instruments. A sax with a lot of small accentuation, breath and so on cannot simply be put on top of an existing track. Create some space for it. Do in the mix what a band would do if the saxophone player will start a solo. Reduce the backing chords to a minimum, dampen the snare , smoother the bass or whatever seems suitable. What kind of sax was it, soprano, alto , bass (I doubt that ...)

Always practice safe sax...........

doubling?