I installed a new (to me) drivetrain on my road bike. It's an Ultegra 53/39 crank, Rival Front Derailleur, Rival rear derailleur, Shimano 10-speed cassette (12-25) and an Ultegra chain.

The chain is (obviously) for 10-speed road bikes and when I fit it to the big ring on the crank and the largest cog on the cassette, the fit was just as it should be. However, as I shift down the cassette, the rear derailleur doesn't pick up the slack and the chain sags. I installed this thing myself and I'm wondering what I did wrong or what I need to adjust to get it to take up the slack.

FWIW, I had the derailleur running temporarily on a Shimano 9-sp cassette with no problems (except for the screwed-up shift increments, of course) before I bought the 10-speed.

Thanks for any light you might be able to shed on my situation.