Stimuli size requested is smaller than 1 pixel on X and/or Y dimensions. Stimuli sized smaller than 1 pixel will not be visible.
This issue is often caused by an inconsistency between the units of your stimulus and the values being requested. For instance a size of 0.1, when the units are height is sensible (1/10th the height of the screen) but a size of 0.1 when the units are pix would not be sensible (1/10th of a pixel).
Check the size and the units of the stimulus carefully. You may also need to check the monitor calibration if you’re using units that depend on the monitor size and resolution (like cm and deg).