The volume of solvent to be evaporated on the rotovap is often underestimated and too small of a round bottom evaporation flask is used for it. Instead of changing to a larger evaporation flask it is often more tempting to fill the flask as full as possible. This is not a good idea because the fuller your flask the more likely you are to encounter problems with connecting it to the rotary evaporator.
The flask needs to be connected at an angle (see picture below) and some of your sample is likely to spill when your flask is full. Even if your sample stays in the flask, the chances of it spilling over when the rotation and vacuum are turned on are high.
To avoid losing product, the evaporation flask that you connect to the rotovap should never be more than half full. For instance, if you have a one liter flask it should only be filled with 500 ml of liquid.
If you have more liquid than fits in half of your flask, evaporate it in stages by refilling your flask when the liquid level is reduced through evaporation, or simply use a bigger flask. This way your flask is never more than half full and you will not waste product or need to repeat your process.