Performance Data

I don't understand why programming the CDU can be so quirky. Sometimes I can enter all the required route and performance information and the speeds/altitudes are automatically populated for all the legs. Other times I get nothing at all until I exit FSX, reload, then go through the same exact steps and it works fine. I have also had problems on occasion arming VNAV on the ground and the same thing fixes it....closing FSX and reloading under the same conditions. It just seems quite buggy. Am I the only one having problems like this?