Things to check: "Trigger 2" is set to on for the 990's inputs (done for each input under the 990's "INPUT CONFIG" menu), the SA amp's main power switch is on, and the SA amp's "Off/Auto/On" switch on the front panel is set to "Off" so the trigger will control it.

It also appears that you can use the "Auto" setting on the SA amp with no trigger cable at all and the amp will come on whenever it senses a signal, much like typical powered subs operate.