For those that don't like to read, just try going over 70,000 meters. For the rest, I had this issue with two passengers who both wanted the same thing; "Sub-orbital space flight on Kerbin" (so not the problem others have mentioned regarding different objectives). In my case, I'd recently modified my passenger craft to add a third command pod, increasing the weight. The Nav Ball says orbit somewhere near 30,000 meters, so I thought that was sufficient, but in the case of at least 2 contracts for 2 tourists each, I had to break 70,000.
