If you pre-wedge before prepping and push the wedge in harder after a few minutes, you will get tight contacts every time. Make sure that the wedge is tight, otherwise select a larger size.
As the teeth separate the matrix will stay in contact with the adjacent tooth since you have bonded it to that tooth. This does not happen with usual clinical procedures since the wedge pushes the adjacent tooth away from the matrix.
Our matrices are made with 35-micron foil. This is only the thickness of one fine hair and our Separator Wedges easily create this much movement. You can confirm this yourself, by pre-wedging while the local anaesthetic is working and then try sliding a matrix between the contact point. You will find that it will pass through, proving that you have separated the teeth by at least one matrix thickness.