FileMaker, Inc. has produced a rather useful printing troubleshooting guide here.
If following that article does not resolve your issue, the problem is likely related to that specific printing device, in which case you would want to contact the manufacturer for a resolution. Dymo's support page can be found here.
1. I adjusted Layout Setup to get each Dymo label to print correctly. Took a while, but got faster as I got better at it.
2. In FMP 9 on Windows 7 64-bit I had to interrupt a script and manually change printers.
BUT...something new!! The Print function was updated (don't know in which version) to specify particular printer.
FMP9: Print [Restore; No dialog]
FMP12: Print [Restore:DYMO LabelWriter; No dialog]
So I now use Print Setup to specify portrait or landscape followed by Print to print to a specific printer. If FileMaker is on multiple computers connected to different printers, Get(HostName) could be used to direct to the right printer.
3. In light of this (undocumented?) improvement in FMP 12, I am sorry about my previous 'poor printer support' remark.