Check the color
Use a dark QR color on the white background. Very light custom colors can be harder for phone cameras to distinguish, especially in poor lighting or after printing.
Keep the clear margin
Leave the white margin around all four sides of the QR code. Cropping that margin or placing other artwork against it can prevent a scanner from finding the code.
Try without the logo
A logo covers part of the matrix. If a code with a logo does not scan reliably, remove the logo and test again.
Export at a useful size
For raster files, use 1024 pixels or larger when the code may be enlarged. For print, SVG or PDF keeps edges sharp at different sizes.
Test the real output
Test the downloaded file, not only the preview. For print work, test a physical sample at its final size and expected scanning distance before producing the full run.
Create a high-contrast QR code and test its download.