Failing to access index

Hi experts,

We have created and activated a custom index on VTTK utilizing fields STTRG, DATEN, SHTYP, and DATBG.  For some reason, I am completely unable to hit this index -- even when simulating SQL SELECT through ST05.  Even if I specify all four of those fields in a SELECT statement, the access path indicates that no index was used. 

Any off-the-top things I should check for this?