Your white ink went through your screen but the result isn't opaque. When printing with light colors, you can see the fabric's color through it.
▶ Possible origins
That is a normal behaviour: printing white or clear colors on dark fabric is the most technical type of one-color printing.
The pigment may be strong, but the fabric absorbs a part and covering a darker color may require multiple layers.
▶ Solution
Make sure you're using a good quality ink, that has a good coverage (like the one we sell in our shop).
Even then, if there is transparency, you will need to print multiple layers to cover it:
Make a first print pass.
Move your screen up slowly, but make sure the frame and fabric don't move!
Use a hot air gun or hair dryer to dry the printed ink for 1 or 2 minutes (stay alert to not move anything, and to not expose the screen to too much heat).
Lower the frame again and print a second pass.
Light the screen carefully to check if this made the coverage sufficient. If not, dry that new layer and continue the process to print a 3rd one.
