The problem is probably related to the mapping channels. In the Vertex Paint modifier Set the Mode to Map Channel and then assign the channel you wish to use (2 in this example)
Then in the Forest Pack Object, make sure that the Transform > Rotation > Map Channel
value correlates with the channel selected in the Vertex Paint modifier.
Live painting should now work as expected. Please find attached a sample file containing this setup.
This also post got me thinking about how to create a hay bale with FP, so I've had a quick go but tackled it slightly differently. From what I can see the majority of the individual hay strands in a bale run vertically. On the top and bottom you would therefore see the ends which get bent over as the bale is handled.
So to do this I've tried an approach that uses 2 separate FP object, one for the sides and another the top and bottom. To create the surface, start with a chamfer box but then split it to separate sides from the top and bottom like this:
You can then use the sides marked in green to create a FP object that scatters vertically aligned straw segments (with a little randomised variation), but for the top I've created a small patch of segments that are stood on end with some strands bent over at the end, like this:
These are scattered on the polygons marked in blue. This should give you the impression of hay running vertically throughout the entire bale without needing to fill the whole volume with geometry.
I've attached a sample file of this setup, in case it's of use.