The first time I changed oil in this vehicle, I had no problem. Except maybe that little round disc with 1/4 inch indentation was over tightened on the filter and the entire filter came off. So I did not have the opportunity to use the plastic drain tube. But when the filter came off, all of the engine oil came pouring out as expected. It was messy, but no big deal. Since then, I have changed oil in this Siena several times with no problem.
Yesterday when I took the little round disc off of the oil filter and inserted the plastic tube, only a little bit of oil came out. So I thought maybe that tube mechanism is messed up or plugged or something. So I removed the entire filter and no more oil came out. The dipstick still registers full. At that point I put the filter back on and ran the engine for a few minutes to warm it up a little bit more. When I tried to change the oil again I had the exact same problem. The only oil that came out was what was inside the filter itself.
At that point, I topped off the engine oil with maybe 1/2 quart. We could probably go another thousand miles or so before an oil change becomes drastically important. So fortunately this is not an emergency situation for us. But I do need to get it resolved. I have been thinking about this and the only thing I can come up with, and this is probably a long shot, is that the filler cap for the engine oil is airtight and will not allow the oil to flow out the bottom of the engine. Kind of like keeping your finger on top of a straw full of water. I don't know. Tonight I'm going to try to change the oil again and this time I will take off the filler cap and see if it drains. I will most definitely post my results here.
Thanks for your replies I appreciate them very much.