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There could be several reasons for this issue:

1. The triangle might not be rendered properly due to incorrect coordinates or vertices. Check if the triangle vertices are within the viewing frustum and if the coordinates are correctly set.

2. The triangle might be rendered behind other objects or inside them. Check the depth buffer and ensure that the triangle is rendered in front of other objects.

3. The rendering context might have been destroyed or lost. Make sure that the rendering context is properly created and active when rendering the triangle.

4. The triangle might be set to render only once or for a limited duration. Check the rendering loop and ensure that the triangle is rendered continuously or for the desired duration.

5. The graphics card driver or hardware might be malfunctioning. Try updating the graphics card drivers or testing the program on a different system to see if the issue persists.