When ineserting any plug-in, make sure the plug-in you are looking for matches the channelization of the slot you're trying to use it in. For example, since Sonic Modulator is available only as a mono-to-stereo plug-in, you can't insert it on a stereo track.

Remember that the plug-ins above the slot you're using also make a difference. If you have an aux track with a mono input & stereo output, with a mono-to-stereo plug-in in slot A, you can't use a mono-to-stereo plug-in in slot B — the input to slot B is stereo, not mono.