is this a newer board? you might try and use a base cleaner to get any weird shit off of it before you add more wax.
i had a newer deck this spring, and noticed the same problem, however it was outdoors. i believe a lot of pollen/tree sap/random shit was getting stuck on the bottom of the board - if i scraped the board with a credit card, i would get some dark sticky shit coming off of it. i did a thorough base cleaning, and after that, i was hauling ass.
i think sometimes the factory wax that is on the board might pick up shit in the snow.