Changing velocity on crescendo/descrescendo

Hi,
I have a problem.
When I put a crescendo or decrescendo over a let's say four notes, it doesn't increase the volume fast enough. A few days ago I saw the playbutton under properties of the crescendo and I could click it to change the velocity. Now the playbackbutton is greyed out and I don't know how to change it anymore. Does someone know how to fix this?