V-belt drain plug

Try some motor oil or grease. The silicone sprays don't work well on rubber and it even swells or makes the rubber soft.

grease should work quite nicely to slip it back in, maybe try using a fine point as well to get certain areas in

Thanks Rollin and Muddin,

I applied some white grease and here's what happened.

I tried and tried to insert this "cone-shaped grommet" without luck, and I walked away (ran out of descriptive words) and had supper.
I went back to it and resumed squeezing, pushing, etc.. and all at once the middle of the (coned-shaped grommet) fell out and the two pieces looked like a plug and a regular doughnut grommet. The doughnut grommet went on and plug went in a minute, and all is right.

Seriously, apparently some mud / grit stuck the plug to the grommet and the whole thing came off, and the grease did it's job!
It's funny,now!