BN7 Top Frame question

[Awgertoo@aol.com]

The back (folding) bow on my wife's BN7 tends to suddenly shift forward 
toward the main (fixed) bow, causing the top to collapse suddenly.  The bow 
lines 
up nicely with the seam in the top and the top itself fits very well and has no 
extra material.  Short of putting some kind of a stay between the two bows is 
there anything I can do to keep the back bow from moving forward so easily?  

Best--Michael Oritt