Playback volume inconsistencies

Two out of the last three times I have loaded existing projects in Ardour, the playback volume has been inconsistent. On a good load, everything is balanced. On a bad load, The drums (.wav files created in Hydrogen) are barely audible with their faders all the way up (no plug-ins active on them). Guitar is very loud (recorded from amp, also no plug ins), bass is very quiet (recorded DI and shaped with Guitarix) and vocals are quiet (using Calf Reverb). Other similar topics seem to find the problem in automation or plug ins, but tracks with no automation and no plug ins are affected. What could be causing this issue?