.jar not available in Kotlin Code

I have two separate projects A and B, both of them are written in Kotlin und compiled to a .jar file using Gradle. Additionally I'm only able to use the generated .jar of A in project B.

Therefore I imported the .jar with Gradle over the FileSystem:

If i now create a normal Java File, I'm able to use all classes from the imported .jar file.
If i create a normal Kotlin file, not a single class from the .jar is available.

Can anyone help me out here on how to get this thing working?

~Simon