Tempo Changes not working

What jeetee said is still relevant. In measure 1 you have a half note, in 35 a quarter note and 136 and half note. All tempos are as you entered in the score. 75 has a dotted half note=75 (=225 bpm for 1/4 note). I see nothing wrong with what MS is doing. Unless you are truley changing the note value getting the beat it is better to use the palette and keep the unit of measure the 1/4 note rather than varying note based upon the current time signature. In 68 I would actually make 1/8 note = x BPM (x = 1/2 of 1/4 note duration desired or stated another way the 1/4 note tempo divided by 2.) since the basis of 6/8 time in this case seems to be the 1/8 note rather than the dotted 1/4 note (as is common in x/8 time) and definitely not the dotted 1/2 note.

Yes to the above :-). I would also like to point out the Inspector can help you sort these things out. If you click a tempo marking and look at it in the Inspector, you'll see the math done for you, with the tempo marking converted to quarters per minute. The dotted half = 75 in measure 75 works out to 225 quarter notes per minute, as shown in the Inspector. That works out to 150 dotted quarters per minute, which is of course what most people would feel as the tempo for 6/8. The half note = 70 in measure 136 works out to 140 quarter notes per minute. That's definitely slower than the 150 dotted quarters (or 225 quarters) per minute that came before.