Before you jump to new pellets, have you adjusted the gun? I've found in my .22 Marauder it likes the JSB 18.3's at about 830fps or so. Gives nice groups at 75 yards, but nothing really worked till I setup the Chrony and did some general testing and adjusting.
CA's recommendation of cleaning the barrel is good as well, you just never know, and yes, it does take 50 plus shots to get accuracy back after cleaning, but don't ask me why.
also, look at the magazine to make sure it's not clipping pellets. As good as the mag's are, sometimes that happens. Hajimoto has an excellent video showing how to clean up a Gauntlet magazine, which is the same design and I believe will fit a Marauder. The cleanup virtually eliminated the pellet clipping.