A 320 Pegasus
 

Can someone enlight me on the operation of the Mcdu in the following situation.
When Aircraft is in a turn or close to the next fix, the Mcdu doesn't accept lateral revision, such as a PBD or PB with crossing restriction. Even in selected mode ( Hdg), the computer doesn't like my typing.....
It will accept it once in level flight and away from a fix.
What is the gouge and most important, can I trick the machine!!!!
Thank you Friends.