>I am using Zebra. Font size is as large as possible.

I haven't used Zebra printers myself but I understand they are their own ecosystem with their own foibles.

Some bar code fonts are optimized for a certain point size range, and don't necessarily work well if printed larger or smaller. Usually, the provider of your font package should be able to tell you if they only support a certain range of point sizes. If you're using an integrated solution from Zebra, it should "just work".

If "not all scanners easily scanning the barcodes", can we assume you've checked the problem scanners for dirt, scratched windows or other problems? Are the scanners all the same make/model, or is it that one make/model is OK but another is not?