Are there multiple pins on the net? It may be attempting to complete the route to a different pin then the one that you think is obvious.
You might try routing 95% of the way from pin #1 to pin #2, and then routing from pin #2 to finish the net.
That is odd though. I just routed across a poured plane and it works just fine.
I can't place a partial route either.
I did notice that the route would place if I zoomed way out to tack it to the second pin. A "target" symbol that appears at the crosshairs when the software feels you are centered over a valid pin, but sometimes it doesn't appear when you are zoomed in too closely and snapped to a grid. When zoomed way out, I guess the target (the center of the pin) grows so that the trace doesn't actually have to be completely centered to find home.
... I explained that terribly. Hopefully, you followed what I was saying.