Turbo 400 shifting troubles

To start off, the trans was shifting fine either in manual or automatic mode. It is a pretty recent rebuild, has about 500 miles on it. If I start out at a stop light in Drive or 3rd, and take off the trans shifts through the gears perfectly. I can take off part throttle or full throttle and it shifts 1-2, 2-3 no issue.
When I start off in 1st, run it up to about 3500 and manually shift to 2nd it does not go into 2nd gear. I can run it up to 5000-5500, and it still does not shift. If I let off the throttle, or blip it a bit, it will eventually shift.
Before I started having this problem, I would run it up to 3500, and shift from 1-2 and get a nice firm shift with a chirp of the tires as it shifted.

Checked the fluid, trans is full.
Va*** line to modulator is connected with no leaks, engine pulls 18-22 lbs of va***.
Checked the shifter and cable, and the cable is connected correctly.
My next step is to look at the valve bod, maybe 1-2 shift valve, but I could shure use some help from one of you guys with more trans experience.

Thanks for the help.

 

Did you pull the governor and check the nylon gear. I had one strip off going down the road and it dropped to low gear running about 45mph. Hummed the engine a bit!

 

And check the weights, make sure they are free also, make sure the modulator hasn't got any oil in the vacuum line

 

If it is only in 2nd/L2 check your pan for material & your front band.

 

Recheck your shifter and cable.

 

If everything works fine but manual 2nd, then your front band is either burnt, broken, or you have a cracked servo. Some years had a plastic servo, they could be problematic.