Generally this means that the pin reader inside the printer can not read the chip. This can be fault of the pin reader inside the printer or the chip (oem) or the control chip.
1) Do you have an OEM cart to test the printer to see if it can read the OEM cart? If so - you can replace the OEM chip inside the vlm cart and see if that remedies the situation. If not you probably need a new control chip.
2) If the printer can not read the OEM cart, then the issue is probably with the pin reader inside the printer. It will need to be replaced. These are inexpensive and available from Epson's consumer parts distributor Compass Micro. But, they are very difficult to replace.
3) You could remove the cartridge next to the vlm and look inside with a flash light and see it the vlm pin reader looks just like the pin reader in the working slot.
4) You can examine the vlm cart and see if the chip holder was not pressed securely down and if inserting it into the printer broke the pin reader.
5) By some outside chance there is a thin film of grease or oil on the chip and you can take some household alcohol to clean it and try again.