There are several possible reasons for this.
The main one is that the soldering is too slow. This causes the adhesive on the foil to fail before the solder has a chance to become rigid.
The foil may not have stuck to the glass firmly. Reasons for this are many, but some are:
- Dirty glass. Make sure the glass is washed and polished clean, especially if you have been grinding, when you need to get all the glass dust out of the pits on the edges.
- Oils from your hands. The oils can be natural or from hand creams. If you have oily skin or need to use hand creams consider cotton gloves for use when handling the glass prior to and during foiling.
- Inadequate contact between the foil and the glass. This can be from both the above, but can also be that the foil was not pressed firmly to all the sides and edges of the glass pieces.
The foil adhesive may be inadequate through manufacture or age. If a test piece does not feel tacky to your finger tips, it is not going to stick to the glass very well.