Because I record rap music, most of the tracks I'm recording live are mono, so I set up a track template with several tracks with my commonly used FX on each (Waves Mercury EQ, compressor, & limiter). I used the mono versions of these FX (when there is one). But now I just realized, if I start with this template, and add a stereo file to that track (such as a full beat for a mixtape song, or individual instrument tracks of original beats) the track that the file is on will continue playing mono even if I remove all FX from the track. Most cases I can fix this easily by inserting new audio tracks and cutting and pasting, but in some cases fixing it can be problematic and the easiest way would be to switch the track back to stereo playback.

Does anyone know what causes this and/or how to fix it?

Set the track interleave to Stereo?

Quick question. Are you saying that you cannot set the input on a track with mono FX to stereo? Which "mono FX" are you using (the reason I ask this is I only think of FX as audio or MIDI, not "mono" vs "stereo")?
 
If you have stereo inputs assigned, a track should record in stereo for you (if you view "All" versus the default "Custom" at the top of the track view, you can expand tracks to see inputs, outputs, etc.).
 
Additionally, if already recorded, you can right click on a mono clip and "convert to stereo" rather than cutting/pasting it anywhere.

@Bristol_Jonesey, you are a godsend, thank you. Had to research interleave, but that was the trick. Solved.
 
@mettelius, thanks for the reply, couldn't quite follow you, but yes, some effects do have a stereo version and a mono version (the Waves Mercury collection has a lot that are like this).
 
I usually use the mono versions on mono audio because I assume they may be lighter on RAM and I've occasionally had funny business with things being a bit touchy when mixing through my interface and a mono track gets bounced to either L or R if the FX are stereo.
 
Apparently when you assign a mono effect, it sets the interweave to mono, and processes everything down a single mono channel. Which is biz as usual IF it's a mono track, but obviously obnoxious if it's stereo.

Sonar only sets the interweave to mono for you if the first effect in the chain is mono. The only thing is, it doesn't update the interweave button (bug). However, if you then flip the switch to mono, it won't let you change it back to stereo as long as that first effect in the chain is mono.

If you insert a mono effect after a stereo effect on a stereo track, the signal is summed on two channels. However only one channel is processed by the mono effect, so you could get phase problems as Plugin Delay Compensation is only applied to the processed channel. So if you're going to mix mono and stereo effects, it's best to keep the mono effect on a mono track and send the output to a stereo bus with the stereo effect on it.

I think I got that right :)

It gets worse. Create a stereo track with stereo data but insert a mono effect and it becomes a mono track, regardless of how the interleave option is set. Create a mono track with mono data but insert a stereo effect and it becomes a stereo track, but with the sound skewed to one side until you switch the interleave. Or really screw things up by using a mix of mono and stereo effects, then post here asking why panning is broken in SONAR.
 
Best to avoid those complications when possible, by keeping mono mono and stereo stereo all the way through from input to mix bus. If you must use a stereo effect on a mono track, set the interleave to stereo even though the data is monophonic.

This thread has been quite the eye-opener for me. I think I have inadvertently triggered these comments in the past but didn't understand the signal flow enough to realize what was happening.

talking of signal flow, someone on the s***** o** forum made some charts:
 

from: http://forums.presonus.com/posts/list/25568.page
these are really useful not just for general understanding of signal flow & gain staging etc, but especially for the given daw...
does anyone know if there's an equivalent for sonar?

There is a signal flow diagram in the documentation

http://www.cakewalk.com/Documentation/default.aspx?Doc=SONAR%20X3&Lang=EN&Req=Mixing.07.html