Epson DURABrite Ultra Ink is OEM in both the XP-340 and the WF-7210. People use the OEM ink in the WF-7210 without the problems you are experiencing.
Which transfer papers are you using? What color shirts? What temperature are you pressing at? How long? How much pressure?
Cobra pigment inks seem to be the most popular. Your XP-340 does use pigment inks but I don't think refillable ink cartridges are available for it. You might be able to refill the Epson-made cartridges but print quality will deteriorate. If you plan to print a lot of transfers it might be more economical, in the long run, to purchase an Epson WF-7210, even if you work out the issues with the XP-340, because of the high cost of OEM cartridges/ink.
Which transfer papers are you using? What color shirts? What temperature are you pressing at? How long? How much pressure?
Cobra pigment inks seem to be the most popular. Your XP-340 does use pigment inks but I don't think refillable ink cartridges are available for it. You might be able to refill the Epson-made cartridges but print quality will deteriorate. If you plan to print a lot of transfers it might be more economical, in the long run, to purchase an Epson WF-7210, even if you work out the issues with the XP-340, because of the high cost of OEM cartridges/ink.