If you have had exactly the same problem with two different derailleurs and cassettes there is a fundamental problem with what you are doing. Any chance the high limit screws are set too tight? If the RD will barely shift into the smallest cog because the high limit is set too close then shifting the remainder of the cassette will be compromised. Try disconnecting the shift cable at the derailleur and shifting the derailleur through it's range by hand while turning the crank. Be sure it's travel is good at both ends, i.e. it goes into the small and large cogs fully but doesn't try to spill the chain. Once that's good, connect the cable and use it's tension setting to adjust the shifting.