Cannot re-boot 16Mb Pico Lipo

Hi - I recently bought a 16MB Pico Lipo, loaded a UF2 (default state when first plugged in) and then tried to load another UF2 using the boot - power buttons. However, it never goes into boot state so I’m stuck with the first UF2 I loaded.

I did solder a couple of headers (no shorts, good solder points) and I tried the GND to RUN short but this still didn’t work.

I also have a 4MB Pico Lipo and it works fine.

Any ideas?

Thanks