spare ribs

If you use a rub with more sugar, a bark will form easier. Also If you use foil, it can soften the bark on your ribs so you may want to try not foiling. There is more fat on spare ribs, so that might be the difference between the bark on your bbs and the bark on your spares. Also you could coat the ribs in yellow mustard before you put your rub on. I haven't done it with ribs but on butts that gives a good, thick bark.

A couple things. Are you trimming any of the fat away on them? What method are you using for smoking them? 3-2-1, 2-2-1, none of the above?
A nice thick bark can be made by either brushing them liberally with plain yellow mustard. You won't taste is in the end so don't let that be a concern.
After the mustard, apply your rub.
Or, like I do, I trim any heavy fatty area down a bit, then apply the rub in layers. (I don't use mustard when doing mine) Meaning, once the rub is on, I let it sit for a bit. You will see the rub go from a dry appearance to a wet appearance when the rub starts to draw moisture out of the meat and such. When it does this, apply more rub and wrap them up in saran wrap nice and tight. When it comes times to smoke, you can apply more rub at that time if you like or just toss them on.

If you foil your ribs, the bark can easily be scraped off do to the moisture effecting it. If you do foil, try making a triangular tent, (closed at the top) so the ribs sit in the foil rather than being wrapped in it. If you finish the ribs unfoiled, its easier to remove them from a foil tent than it is when they are wrapped up and you won't disturb the bark.