javap will not find classes that are on the classpath unless the class is in the last entry.
eg.
>javac -d c:\myclasses foo.java
>set classpath=c:\myclasses;%CLASSPATH% // where %CLASSPATH% is not empty
>javap foo
>ERROR:Could not find foo

however
>set classpath=%CLASSPATH%;c:\myclasses
>javap foo
will succeed