Fix My ChessMaterial walks off the board in positions you understand — a loose piece left for a single tempo.
A piece sat where it could be taken for nothing — undefended, or guarded by something worth more than the attacker. A whole piece is roughly three pawns; give one away and no plan survives it. The fix is a 5-second blunder-check on every move: “if I do nothing, what can he capture for free?” Look at your own loose pieces before you commit.
The good news: it is a pattern, not bad luck — and patterns can be trained. Step one is measuring yours: how often it appears in your real games and how many rating points it drains. Then the daily repair: 5 minutes on your own positions, not invented puzzles.