Window caulk gets a gap in winter
Sometimes pure silicon will not stick well to vinyl.  I suggest you use a siliconized Acrylic caulk that matches color or could be painted if needed. Be sure to force the caulk into the void with your gun, then round it off smoothly. Be sure to fill deeply into the gap between the window unit and jams. If they are recent model vinyl cased windows, there is no reason to consider replacement.