I used to fight poor sealing performance like this nearly every ride. On my XRR I found the solution was to replace the spacers with hardened ones and replace everything at the same time and fill every nook and cranny with waterproof grease. Then replace everything altogether.
On my 04 EXC I got the best results from regular maintenance. Cleaning out the old grease and getting new stuff in there. The spacers I had weren't hardened but keeping good grease in there helped alot.
Now on my 08 EXC the spacers were updated along the production line between 04 and 08 and have a much better sealing surface and I haven't had a hard time yet but I still take the axle out and clean and grease it after every couple of rides and after EVERY muddy ride.
Throughout this I had some premature bearings that let go way too early but my biggest incentive was stuck axles. Its no fun working on stuff when you can't even get the wheel off