My suggestion, I don't have Buildtak and no experience with Flux soft plate but if I were you, if the failed print is very thin, like about 1 or 2 layers thick, I would not clean it. The next print you make, slow down your print speed and try a simple geometry like a square. It should stick on it and hopefully they both will come off. I did that on occasion on PEI type surfaces. Again this is just a suggestion only!