Ive got one of these and it can be a little tempremental at times.
I know this sounds patronizing, but make sure the card is inserted into the slot the right way.
From what it seems, all of Neo Flash's hardware runs off Gameboy Advance Carts/hardware, so if you look closely, youll notice one end of the PCE card is actually a GBA sized/pinned end.
You can plug it into the GBA, but nothing will happen. However the card reader writes to (if im not mistaken) it in GBA Mode but to a format the PCE understands.
Good luck!