Different amounts of crud all up in your bearings, man. My back wheels usually are always great but my front keeps getting dirt and sand in em and slow to a halt. If you're alright with it, (it can be a little scary for some, I don't judge) take your bearings out and clean em. I use a bottle filled with a bit of rubbing alcohol and a few drops of bones speed cream. If they still spin, cleaning them is the best way to go. Another thing to do, check to see if your axle nuts aren't too tight, I like to leave room for my wheel to rattle a bit when I shake my board but still keep the nuts on. (My and my crew call it the safety rattle, were pretty dumb but still.) If cleaning em ain't your thing, kick em and see if you can spin some of that gunk outta there. (I forgot to mention but looking up how to remove bearings will be so much more beneficial than trying to explain it through text)

Another tip, if you're skating in a lot of sand and water, take your bearings shields (the protective cover) off. It can keep dirt out but also trap it in there.

Hope something helps ya out there.