Occasionally we see instances where the printing is not sharp. Short barcodes are often still readable but longer barcodes fail to scan.

 It could be that the printer has a physical issue like the thermal strip needs to be cleaned or replaced, but most often the cause is that the printer resolution is not set to the "native resolution" of the printer. The pdf manual for your printer will likely let you know what it should be set to. Often the native resolution is 72 dpi for Zebra thermal printers, and printing is blurry when they are set to the max dpi of 300 or any other resolution. 

Another fairly common printing issue is that part of the barcode gets cropped off. This will prevent it from scanning.

It's difficult to tell when the barcode is getting cropped since humans are terrible at reading barcodes. The scanner, on the other hand, can tell, and if even one line is missing or modified it will refuse to read the barcode. The solution is often to modify the template where the barcode is being generated so that it has sufficient horizontal space for the value being entered. 

Barcode values can be too long to scan.

Regular UPC codes are 12 digits and tracking number barcodes are often 14-20 characters so anything in this range will likely work fine. There's no maximum length or limit for most barcode formats. Instead, the maximum length is limited by a number of factors like the printing resolution, the horizontal space of the barcode and the ability of the scanner to read it. 

If you are generating barcodes from your SKU and you have some very long SKUs, it would be advisable to reduce their length to 20 or fewer characters to maximize scanability.