Some sample code would help and my test seems fine. I'm guessing maybe use variables to debug, perhaps clear the screen or try another font.
GLOBAL string$ = "This is a test", screenwidth, screenheight; GETSCREENSIZE screenwidth, screenheight
WHILE TRUE; CLEARSCREEN
PRINT string$, screenwidth/2-LEN(string$, TRUE)/2, screenheight/2
SHOWSCREEN ;WEND