Are N64 transfer packs notoriously unreliable?

I randomly picked up two different units to transfer my data from mario golf and mario tennis on GBC to their respective N64 counterparts, and neither transfer pack successfully started the transfer process for either game. Is this a known issue?

I don’t have any other transfer pack compatible games to test with. I was just curious whether anyone else had a similar experience or knew if these things are fragile.

They are very picky. I have never gotten either of mine working while an N64 controller is connected via extension cable. And most of the time you really want the transfer pak to be un-nudged or the connection fails.

I just got one to work. Yea, very picky indeed. I did nothing different. I didn’t even clean the cart and it suddenly worked.