Dr. W. Edwards Deming noted that "the greatest losses are unknown and unknowable." When a person like you becomes so frustrated that he or she no longer makes suggestions, the firm is losing in ways they cannot even begin to calculate. You have only two choice beyond non-involvement: wait until some shift occurs. It may be a new boss, a merger, perhaps even your promotion. The second choice is to continue proposing ideas on a periodic basis. Amuse yourself: try different time, different techniques, different choices of words. You may even consider approaching your boss as a team. There's undeniable strength in numbers.