Recentley, I have been unable to hear my guitar as I try to play along with other tracks.
 
1. drop a loop into track one, could be a wav, or a midi pattern routed to a synth.
2. record a bass track via midi routed to a synth.
3. Add an audio track, to audition my guitar playing, enable input echo on; everything is copacetic.  Add amp vst to effects, still hearing the good stuff.
4. Press play, the bass and drum go off on their merry way, whistling, but the guitar is now inaudible, the meters for the guitar track impotent.
5. Press stop, guitar is now again audible.
 
additional edit after first response below
 
6.  Record arm the guitar track, I get sound, which is expected.
7. Record arm the track, press play, no sound, which is not expected.
8. Record arm the track, press "R" on the keyboard, or in the control bar, get sound, which is expected, but I do not want to record.  I would like for step 7 to make the guitar audible.
 
I used to be able to play recorded material and play along, but now, it's a no go. 
 
Any suggestions?
 
I use a Scarlett 18i 20.  2017.05 build.
 
Jesse Q. Screed

Make sure the "record" button is pressed on the guitar track?

Hey, I figured it out on my own. It took a while, but I did.  Kudos to BRainbow for at least trying.
 
The answer was in Preferences, under playback and recording.  Somehow, "Disable Input Monitoring During Playback"  was selected.  I don't mess with the Preferences often, because I check those each time I do an update, which isn't often.  So how  "Disable input monitoring during playback" was checked I'll never know.  Maybe I'm a sleep walker.
 
Jesse Q. Screed