IllegalAccessError, when running from JAR

[Java and Swing]

I have an application which loads some Java code from a JAR. So I have
two jar files....

MyApp.jar and Blah.jar.

MyApp.jar is the main application, and at some point it uses a
ClassLoader to load Blah.jar, and instantiate a class inside of it.
However, for some reason when I run MyApp from the jar, as such:

java -jar MyApp.jar .....I get IllegalAccessError: tried to load class
from ......etc.

However, if I run it from the class files as such:

java -cp bin;Blah.jar com.acme.foo.Main it works just fine.

Any ideas what the problem is? how to fix it?

thanks