Plastic tensions jambs...
Know what brand the window is?
There's most likely a spring loaded cord with a stainless steel hook, which atatches to the sash. (Called a sash balancer)
Obviously the weight of the sash is heavier than the rating on the sash balancer.
Do the windows have insulated glass in them?
If so, did you recently have a broken sash glass replaced?
(Maybe thicker glass was used, which would increase the weight)
I've been away from the glazing trade for two years now, but do remember seeing this situation on new retrofit packages designed to replace older wood sash double hungs, which included two new sashes and the jambs which had concealed sash balancers behind the visible jamb.... Can't remember the manufacture's name though.
If you had them installed, I'd get in touch with the installer... He has access to the manufacturer and hopefully, they offer a higher rated balancer.