The same story happened in my previous airline.
They were very satisfied to see a few experienced and expensive pilots leave... until too many experienced and not so expensive (after all) pilots left.
By then it was too late. Not even a 30% raise would stop the flow. Only a 50% raise stopped the ones who hadn't left from looking for another job. Once your employees are in the process of finding a better job, there's not much you can do to stop them. You need to stop them before they apply elsewhere, possibly even before they think about applying elsewhere.