Can I use devices with different unit loads on the same bus? If not, why?
EDIT - are you using A and B the right way round? Is it wired correctly on the sparkfun unit to how you've wired it on your own modules?

If you aren't using terminators then you should. I've attached a drawing to show what you should do - this makes it a little difficult to put as a comment.



Quite possibly, with the extra loading of the sparkfun breakout board you are just about able to reduce data reflections to an "OK" level.It could also be that the sparkfun output is slew rate limited and this would reduce the effect of reflections.

What type of cable are you using i.e. twisted pair with Zo = 120ohms, possibly screened/shielded etc..