Threading

Hi
I got my lathe Threading today.
I wrote a small program to test the encoder and make a 4 pass thread.
1" with 20 tpi. After the last pass a message came up saying that the
z axis could not keep up. Ran the program several times and each time
at the end the same message came up.
I increased the accel,max speed etc. and there was no difference.
Tried slowing the spindle down and the same results.
If you have any ideas please let me know.
eddie