RWS makes a product called the "Ultimate Pellet Assortment". There are five compartments in the container, each containing 100 .177 caliber pellets. I would suggest ordering a packet and giving them a try. One of the pellets in this assortment may help with accuracy.
If you can find/use the specific pellet Umarex recommends, that may also be a solution. My Daisy rifle shoots the RWS Meisterkuglein best....the Marauder likes the Crosman "Premier". It's really amazing how much difference the correct pellet can make. Daisy model 853 and 6 shot 50 foot group below. Very inexpensive Barska variable scope, RWS pellets. Rifle was purchased used from the CMP website....not pretty but you get a Walther barrel for $105.00 + shipping.