Sometimes, it's a matter of materials or manufacturing. Occasionally, it's a design issue. But more often, it's user error. Regardless, sometimes our crossbows don't perform as well as we'd like. Below, I've listed five of the more common issues crossbow shooters might encounter, along with their causes and solutions.
Crossbow will not cock: This is less common than it used to be but could well be the result of not having the safety mechanism in the "fire" position while cocking. That used to be necessary on some crossbows, but for the most part, it no longer matters whether the safety is on safe or fire when cocking. In that case, make sure you are pulling the string all the way back.
Safety did not engage while cocking, and crossbow will not fire: In all likelihood, the bow was short-cocked. In other words, the string wasn't pulled all the way back, far enough to cock the bow and engage the safety mechanism. You should be able to simply re-attach your cocking mechanism and pull back farther, until you hear a distinct "click" and see that the safety has engaged.
Safety did not engage while cocking, but crossbow will fire: It shouldn't happen, but I've encountered this on bows from several different manufacturers. One solution is to simply click the safety on after cocking and continue as usual. However, something is not working properly, and I would advise taking the bow to your local pro shop or contacting the manufacturer or place of purchase about possible repair or replacement.
When cocked and loaded, crossbow fails to fire: It's possible that as in the previous example, the bow was not fully cocked. However, a far more likely cause is that the bolt is not pushed all the way back, or has slid forward. I've encountered this issue several times with my fishing crossbow, probably because it spends so much time pointed downward.
All things considered, this is a good problem. What's happening is that the bolt is not seated far enough back to make contact with the string and, more importantly, disengage the anti-dry-fire mechanism (ADF). It's a good thing, because were that not the case, the bow could fire with a bolt in place but not in contact with the string. The results can be as bad or worse than a dry fire. I've seen it happen on a crossbow where the ADF malfunctioned. You should also check that the arrow-retention spring is in good working order, as this piece is often connected to the ADF.
Crossbow fails to group well: There are several possibilities here. So, let's go through a few of the most likely. First, check all the screws, nuts and bolts holding your optic in place. If anything is loose, tighten it up and your problem is probably solved.
Next, make sure you are using the correct cocking device, which means the one that came with your bow, or one that is specifically recommended by the manufacturer. Many look alike, but not all are the same. Barnett, for instance, has different models for different crossbows.
Once you've determined you have the right device, make sure it is properly centered before pulling back. The most common type consists of two hooks. Ensure they are hooked on the string and each is flush up against the rail. If one hook is farther away from the rail than the other, it will result in unequal pressure pulling the string back. As a result, the string will not be centered and will exert more force toward one side, possibly causing a slight reduction in accuracy.
Third, make sure you are using a bolt with the proper spine for your bow. You can overcompensate – use a heavier bolt with a stiffer spine – without too much of a problem. Going the other way – using a lighter, less stiff bolt than recommended – can be more problematic, particularly with faster crossbows that transfer more energy to the shaft. When in doubt, check the manufacturer's recommendations.
Fourth, check to see if you are using the proper nocks. Crossbow makers may recommend a specific nock type (flat, half-moon, etc.) for a variety of reasons, such as a long power stroke resulting in a steep string angle at full draw or the claw pivot being above or below the rail. Using the wrong nocks could also result in more serious issues than poor accuracy.
When in Doubt
As noted, these are some of the more common issues and answers; there may be others. If the solutions prescribed don't work, don't panic. Take your crossbow to the local pro shop and have someone look at it. Chances are they've encountered your issue before and have several options on how to fix it, the right way.