Try this:
1) Press Ctrl+o to open the preferences window.
2) Select the Player tab on the left side.
3) Toward the bottom, make sure the "normalize volume of tracks with replay gain tags" box is checked. Smart Gain only lets MB decide when to use the Album or Track gain value of the replay gain tag, you must have replay gain active as well.
4) If this doesn't help, make sure you have the latest version of MB installed.
Home Desk ~ MB 3.3 Portable • Questyle CMA400i (ASIO) • Sennheiser HD 660S (balanced) / Audeze EL-8 Closed Back / Fostex TR-X00 Ebony • Teac AI-101DA • Jamo C93 + Dayton Audio SUB-1000
Work Desk ~ MB 3.3 Portable / Tidal • SMSL SU-8 v2 • Nobsound NS-05P • THX AAA 789 • Sennheiser HD 58X (balanced)
OTG ~ FiiO M11 • Audiofly AF180 / B&O H6