The main thing that can stop your board from rolling is bearings that are on their final leg.
There are many differences between all the bearings that are on the market today. They all feature 6-7 balls with a nylon cage enclosed in a metal race to hold them in place. Add a shield or two, and that is what prevents the bearing from the elements, being dirt or grime. Some are easier to clean, and others are better to just replace as you feel them slowing down, but you get what you pay for in the life of a bearing. The more expensive bearings do not mean they roll any faster, except the difference of less balls in the bearing, but you are paying for the quality of the bearing and how long it will last before it is done.
There are two styles of bearings with the shape, the standard bearing and then the tophat style bearing. The Top Hat style bearings are a newer trend in the longboard industry, with a reason behind it. The Built In (Top Hat) bearings are a standard bearing, but with a speed ring on the outside and half a bearing spacer on each bearing, giving it the look of a tophat. The way that this works is that it decreases the slop in your bearing, which essentially makes for a smoother ride or slide. A few different bearings that fit this criteria are the Magic Shrooms, Bilt-Ins, Luxe Built-in Spacers, and the Seismic Tektons.
The standard bearings come in a few different varietys. There are double shieled bearings, like the Magic Race Bearings, Bones Swiss Labirynth, and the Rayne Stoopid Fast. These double shielded bearings stay cleaner longer, but are a little bit more trouble to clean because of the added shield on the plastic race side. The single shield bearings like the Bones Reds, Bones Super Reds, Bones Super Reds Ceramics, Bones Swiss, Bones Swiss 6, and the Bones Swiss Ceramics are easier to clean, but can get dirtier easier with the open face on the inside.