window will not stay up

You chose the 18-032, top sash guide, to ask your question so I am assuming you replaced non-tilt block and tackle balances.
The problem could be the top sash guides, such as this 18-032, are broken and the balances are shooting up past the window sash.
Or the problem could be the strength of the balance, remove the window sash and place it on a scale. We need the length of the actual window balance (exclude the balance attachments in this measurement) and the weight of the window sash in pounds to determine the proper stamp option.