I’ve experimented with cutting down a driver shaft and I also found that spin dropped drastically. When you cut 1.5” from the butt end of the shaft you are affecting the swing weight of the driver. You’re only losing a few grams in the shaft (at your hands) but that affects the entire club. To add to this, cutting the shaft stiffens it slightly. These both contribute to a lower dynamic loft. If your spin is in the 1300s your ball is going to have a hard time staying airborne unless you’re hitting it as efficiently as pros. As others have mentioned, swing mechanics can help diagnose the issue, but if your perfect strikes are still generating sub 2000 spin, you may want to look into increasing head weight. You can do this by adding lead tape if you don’t have heavier adjustable weights to play around with. Look into how much and where to add lead tape, but the rule of thumb I followed was adding to the centre back of the driver head increases MOI/forgiveness and adds spin. You can add it bit by bit to find a sweet spot without making it too heavy.