pin him down ...
if you can't get flowcharts, get reqts/interface spec ...
maybe you are trying to debug his code? let him debug it! ... just match the h/w to the specs and test that it matches the specs .... ask for his test cases instead, and see if a test case matches your problem ... hope he is not your tech lead :) ...
or if he has got a good nice set of tests that DO match the specs with no test errors and no errors in the tests maybe it IS in the hw ... happens sometime ...
james