how do I fix "warning: crescendo too small"?

[Michael Kiermaier]

I get the warning
"crescendo too small"
for two notes close to each other, both with dynamic marks and connected by a 
crescendo. In the pdf output I get a very degenerated crescendo mark: Only a 
vertical line.

To fix this I have to force the two notes further apart. But I don't know how 
to do this.

Can anybody help?

Thanks,

~michael