In Sibelius 4 i am entering the code H for a creshendo and using the space bar to extend it.  For some reason, the creshendo does not playback and just jumps to the next dynamic when the playback gets to that measure. I dont know how to make it so the playback uses the creshendos that i have put in.
Any Solutions?

I'm not sure if this the correct answer to your problem, but it is usually the quick fix for me:

Open your properties window (ctrl-alt-p).
Click on the tab marked playback.
Click on one of the [i]crescendos[i][/i][/i] you want to increase.
Under Hairpin(in the prop window), change the default 10% to something more like 40% -- you can play around with this value until it fits what you are wanting to hear.

If that's not it, I don't really know what to tell you.

Good luck either way!
ER

Yep - Eric is right. The only other thing that may be necessary is if you're entering the hairpin beneath an accent/tap pattern. This typically negates the accents, so you'd have to manually lower taps using the ";live playback velocities"; feature in Sibelius.

One minor problem, i cannot seem to find the properties menu, and i have looked everywhere. I'm on a mac but i still put in that shortcut and i got nothing.

Without looking directly at Sibelius, you should be able to open the ";windows"; menu and select it from there.

Rhetorical Question Time: So, if you are using a Mac, would there actually be a menu called windows?

Good luck

If you are trying to playback crescendos/ decrescendos on long sustained notes (rather than busy rhythmic patterns) you will have to create a hairpin and use the ";Cresc/ decresc playback"; function from the Plugins menu. Go to Plugins, then Playback, then Cresc/ decresc playback. This is good for timpani rolls, mallet rolls, or any hornline parts that you may write.