There's nothing wrong with your printer. Both your printers use the same thermal technology but are still different from each other in few ways.
Like for Eg. some printers use100 g's permeter and others use 85 g's permeter. I am not pretty sure which category your printers fall in but, what I do know is the 100 g's permeter papers can be used in printers that use 85g's permeter papers, but strangely the 85 g's per minute printers cannot be used in printers using 100 g's permeter papers. It is confusing I know but worth knowing. You can run Microsoft Office in Mac, but you can't run Mac in Windows, it's just that concept. Purchase only that paper next which suits Your printer best, that's all.