What you describe sounds like Buss/Master/Group Compression gone wrong.
This effect sometimes appears when you have a compressor set to affect both the bass and the lead (via any of the Buss/Master/Group)
When the bass hits the compressor the really lows(which consume a lot of space) instantly bring the threshold down and lower the volume of each other affected track.
The reason you might not be able to hear it, is your monitors. If your monitoring system can't produce the really lows , you feel like you have to boost the bass in order to be able to hear it , but you end up boosting the very low frequencies that you can't hear thus driving the compressor to a dive.
Some times this effect is wanted by many producers as an asphyxiation effect.Like when the bass hits it's so loud everything ducks but it's done on purpose and very carefully.
To see if this is your problem check out your routing carefully , remove all master/group/buss compression/limiters and listen again.
If you don't use group compression this could be a frequency problem. What you can do is:
- Find the frequency footprint of your lead synth!
- Insert a sidechain multiband compressor in your Bass channel
- Compress the synth frequencies out of the bass when the Synth plays.
This will clear out the space for the synth only in the desired frequency range and only when the synth plays!
It's really hard to understand without a sample.
I hope this helps :)
P.S. Don't try to follow the 2nd solution if you haven't first ruled out the 1st! or else if the 1st is the problem , the 2nd will not make any (or at least any significant) difference!