Republican Sen. Paul S. Trible Jr. surprised Virginians by announcing that he will not seek a second term because "I want to see more of my family." Trible, 40, explained in a two-minute television commercial that he had been home for dinner only three times in a recent two-month period. He also cited job frustration, saying that "for all the Senate's greatness, much of the important work of the nation doesn't seem to get done." Trible, who served on the Iran- contra panel, was chairman of President Reagan's 1980 campaign in Virginia. He was facing a tough reelection campaign, with the Democrats expected to run former Gov. Charles Robb.