Trombone note in lower register

When I write a trombone note an octave lower than middle C, (I believe that's c3) it does not play the correct note. In bar 3, it is a half-step or a whole step lower. For example, in the file I am sending, the C3 under middle C sounds like a B natural. In bar 3, the A sounds ok. In bar 4, the D sounds ok, like a true D. In bar 5, the C sounds like a B natural. In Bar 5, the C and the B flat sound almost the same. Bar 6 sounds ok. In Bar 7, the "a g f e" sequence sounds ok. In Bar 8, again the C sounds like B flat, but the A3 sounds ok.
When I go into the mixer and change to a grand piano sound, all notes sound right. What is the problem with the trombone sound in the lower registers? How do I fix it?