You can cover the area with another type of seal that only attaches to one side only. I would need to see the window to tell you what type but sounds like a simple fix. If the condensation is on the outside it is possible that no leak exists.
home depot sells “thermo-king” peel & stick insulation – it can be separated into smaller strips
When the window is unlocked, can you wiggle it from side to side? It may wiggle a tiny bit but if you push it all the way to one side and you have more than a quarter inch gap on the other side than its excessive. If you look along the sides in the window tracks there should be a couple small holes(more or less depending on how tall the windows are). You will see tiny holes with a small screw in them. When you tighten the screw, it pushes out a wheel against the old window jamb, this will push the new window jambs in tighter and it will tighten your windows so they don’t wiggle so much between the jambs. Also, they may not have insulated well enough before they put the replacement windows in.
Check to see if the weather stripping is still there. It should run across the whole sash and down each side on bought. Most replacement windows have a interlock at the meeting rail (top of bottom sash bottom of top sash) If yours do not try using a heaver weather stripping. Go to a lumber yard that carries many brands of windows they may have something to fix your problem. With out seeing the window I can’t give you a better answer than this.