Gap below bubble seal

Hi, our windows are wood and about 15 years old. Recently I noticed a gap between the bubble seal and the window threshold on one end of the window bottom. The window threshold is newly replaced and properly installed. I noticed the window itself is slightly higher on the left side. Hence the quarter inch gap on that end. I will order new bubble seals, but don't know if that will solve the problem. It obviously is affecting our heating bill. So I would appreciate any suggestions. Thank you, Mrs. Walker