Do not listen to asilva23, this person has no idea what they are talking about.. Common sense?? You fail to have it.
Your hub is made of a certain width to be run parallel in your dropouts, by adding a spacer , you throw that off..and not only are you doing that, you'll be stretching your forks as well. If one spacer is added to one side, you must do it to the other so both ends are even.This however will stretch your forks further. This is a stupid mod for those who have no proper skill in bicycle mechanics or maintenance. Its already been said he does not build his own wheels, so why listen to someone who doesnt instead of taking advice from someone who does?
This modification will result in failure. Your hub along with your forks are not designed to be ran with a spacer on the inside of one . You will damage your forks, you can damage your hub, cones ...etc and your wheel will not be perfectly aligned.
Please take my advice from someone who actually knows what they are talking about and has experience with wheel building. The easiest thing you can do right now is
1- if running a high psi, drop it down some.
2- take the wheel off and put it back on, make sure its fully in the dropouts and turn the bolts to tighten it evenly on both side. Finger tighten them first, then use a tool. It does happen when tightening the wheel can shift slightly.
3- Depending on who built your wheels, how true they are, if the dish is off...it can be a problem. It could also be your tire along with the wheel. This is another that has worked for many. Remove your wheel and tire, the way the rim is facing, flip it around...So lets say if you had a marking on the rim and its normally on the right, you now want it on the left. Properly mount your tire as you did before and see if that helps.
Everything I listed is simple and very easy to do. In most cases, it works. Go around and ask everyone who rides if they have a spacer on side, thats in between the hub and dropout...they will say no.
You can already see from how asilva23 responds, his wording...He clearly has no intelligence and I have no doubt he will respond to this in hoping to justify his stupidity. Do I care? No, because he's an idiot ( you can see its not my opinion, multiple vital user have stated also) who clearly doesn't even know how to properly build his own bicycle.
In all, you may also just have a tire that is to big for your forks. Please try as I have suggested and let me know how things work out.
REFS-
Vital- KyMike, FEDEXdoobie, adrenalinebmx ,MaximusBikes
Bmx-forum-Tumitama, archerbiker9(2x), mrsanchezz, nguyenna, JoeyBones, fast-and-loose and many others
Also multiple sales on global-flat.com and PinkBike. Theres to many to list all. Most sales are done through texting I havent been able to keep track of who is who.