I posted a question about my just purchased 2001 SSEi being slow off the line a while back.
I've discovered that it isn't shifting into 1st gear. I've changed the fluid and added Seafoam over a week ago with no change. What could keep it from shifting into 1st? Could this be a simple fix, or is it a sign of bigger problems?

Does it just have 2nd and 3rd gear? If so, this is what the symptom check says on that.

Definition" The transmission only provides second and third gear in Drive range. The shift pattern as the vehicle accelerates from a stop is 2-2-3-3. Second gear ratio is 1.568:1 and third gear ratio is 1.00:1

Control Valve Body Assembly
A faulty 1-2. 3-4 shift solenoid valve assembly is stuck on.
A missing or damage shift solenoid valve O-ring seal.
Plugged or damaged solenoid valve screen/seal assembly.
Debris is found in the control valve body or the spacer plate assembly.

Case Cover Assembly
A sticking, binding, or missing actuator feed limit valve line-up.

As you've already tried the simpler things, it sounds as if you'll be making a trip to the tranny shop.