Orbital rendezvous with the ISS has a very tight window (2.5-10 minutes), since on-orbit propellant is very limited. They could add more, and make the window a bit bigger, but that subtracts directly from cargo, which is already small: just 1000 lbs.
The launcher has to drop Dragon basically right underneath the ISS; it approaches by raising altitude just several km. (In orbit, speeding up or slowing down means a change in altitude as well, so catching up or slowing down requires a lot more fuel.)
Since the second stage drops off Dragon just below and a little behind the ISS, a second burn to raise the orbit would necessarily take it past the ISS. NASA is, understandably, very cautious around the ISS. I'm a bit surprised the even allowed a secondary payload boost. It was probably on very, very tight conditions.
Making up for the lost engine probably got them to about the right place, but the different flight profile meant it wasn't the perfect place. Presumably it was inside Dragon's maneuver window, but outside the parameters NASA set for the secondary payload. This could happen if the second stage got too far "ahead" of the ISS due to a longer, lower burn, putting the second stage stack immediately below the ISS instead of below and behind it. Dragon, being in a lower (slower) orbit, can just wait a bit and get in the right position, but the booster could not proceed immediately as it would go too close to the ISS and could not wait as it is battery powered. Thus: secondary payload doesn't get its orbit.
Missing the secondary's orbit is unfortunate, but that is the nature of the secondary payload. You take a gamble in return for a discounted ride. It's appropriate for a prototype, as this is, and they'll probably still get their primary mission done--testing that it functions properly in orbit. Probably won't get to use it in their constellation if it works, which would have been gravy.
The launcher has to drop Dragon basically right underneath the ISS; it approaches by raising altitude just several km. (In orbit, speeding up or slowing down means a change in altitude as well, so catching up or slowing down requires a lot more fuel.)
Since the second stage drops off Dragon just below and a little behind the ISS, a second burn to raise the orbit would necessarily take it past the ISS. NASA is, understandably, very cautious around the ISS. I'm a bit surprised the even allowed a secondary payload boost. It was probably on very, very tight conditions.
Making up for the lost engine probably got them to about the right place, but the different flight profile meant it wasn't the perfect place. Presumably it was inside Dragon's maneuver window, but outside the parameters NASA set for the secondary payload. This could happen if the second stage got too far "ahead" of the ISS due to a longer, lower burn, putting the second stage stack immediately below the ISS instead of below and behind it. Dragon, being in a lower (slower) orbit, can just wait a bit and get in the right position, but the booster could not proceed immediately as it would go too close to the ISS and could not wait as it is battery powered. Thus: secondary payload doesn't get its orbit.
Missing the secondary's orbit is unfortunate, but that is the nature of the secondary payload. You take a gamble in return for a discounted ride. It's appropriate for a prototype, as this is, and they'll probably still get their primary mission done--testing that it functions properly in orbit. Probably won't get to use it in their constellation if it works, which would have been gravy.