WALPOLE - Lucette Nicoll eyes the busy cashiers at the Big Y supermarket here. The self-checkout lanes are free. She pushes her brimming cart toward one. Employee Jessica Campbell knows in a glance that Nicoll will need help. Moments later, as if on cue, a light flashes. Service stalls. Campbell steps in. Produce is the culprit. Checkout problems like this are among the reasons Big Y Foods Inc. announced plans to eliminate self-checkout machines by the end of the year. The company, based in Springfield, will add more clerk-operated lanes to the affected 30 branches of its 50-store organization. “We’re very service oriented,’’ says Big Y spokeswoman Claire D’amour-Daley. “There were too many variables where the self-checkouts were not working.’’ If you want to get out of a store quickly by checking yourself out, there’s nothing more maddening than the line slowing to a halt. Self-checkout is typically of those at Big Y: Consumers face a pay panel and display screen, then scan items over a glass plate sensor that doubles as a scale. Items are loaded onto a conveyor belt that ends at a bagging area. What can go wrong? A lot.