why cannot I step into few services while debugging my unit tests?

I am debugging my unit test that cover web-services. I am able to step through some services but not others while debugging my Unit test. All the Unit Tests are successful. Any suggestions? I am using IntelliJ, SBT and services in Scala. Any suggestions/checklist would help.

Here is the version of IntelliJ although I think it is to do with something else. IntelliJ IDEA 2016.1.2 Build #IU-145.971, built on April 29, 2016 JRE: 1.8.0_76-release-b47 amd64 JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o