If copper foiled pieces sit out for any length of time after foiling they will oxidize. This means that the solder will not stick to the foil, as it requires a clean surface to attach to.
Clean the foiled pieces with fine steel wool, pot scrubber or flexible mild abrasive. Make sure you do not damage the foil or pull it up from the glass during this process. It is likely that the adhesive holding the foil to the glass is not as strong as it once was.
I do not recommend using a stronger flux to overcome the oxidisation, as this is often highly acidic and may damage the glass.
Once cleaned, you can flux the foil and proceed as normal.