Some days we have off-days and don't shoot well.
I shot a Benelli with a od once and could only wound a hen pintail - then had to run her down after shooting my last 3 shells at less than 30 yards on her...
I found that my 870 shoots good with mod but kills 'em deader with full. The full choke does miss birds the mod would not but the ducks that fall are rarely kicking with the full choke.
Pattern your loads and try a full choke while you are at it.