Check your tuplet definition to be sure you are asking for three eighths in the space of 2. If you use CTRL-3 followed by a quarter/crotchet the program will assume you want three quarters in the space of 2.

In such a case, I would probably type CTRL-3, then an eighth, then one or both of the two sixteenths, and then click back to change the initial eighth into a quarter.

Many thanks