If you're trying to pattern a feature, such as an Extruded Cut, and not a Body (and no, they aren't the same), and having problems, try selecting "Geometry Pattern" under Options. If that doesn't help, feel free to post it on the forum as a new Discussion. Please include the Part when you do (see How can I attach a file to a forum post?) . If, on the other hand you are trying to pattern a body, read on.
(In the interest of simplicity, the following discusses Patterns, but the same applies to Mirrors.) If you're working with a multi-body Part and trying to pattern a single body or combination of bodies, be aware that "Features and Faces" (red arrow below) will be selected by default when you select the feature. You will get better results if you de-select that box and select Bodies (purple arrow) instead, then select the body or bodies to pattern. Failing to do that is a common mistake.
If that's the situation, and you've already created the pattern, you can edit it if you're using SW2015 or later. You'll need to un-check the Features and Faces box and clear it's selections, then proceed withBodies. If you're using an earlier version you'll need to delete this pattern and create a new one, this time choosing Bodies instead.
If you've done that and it's still not working right, make sure this body hasn't been inadvertently merged with another body or bodies. Click on the feature that created the body, select the "Edit feature" icon, and make sure the "Merge bodies" box isn't checked. Since that box will often be checked by default that's also a common mistake.
Speaking of patterns, I never use sketch patterns. I've found that it works much better to create a single feature or body with the sketch, then pattern it. There may be situations where this isn't true, but I have never run into one. I know there are some people that are hung up on keeping the feature tree as short as possible, but sometimes it pays to have the extra line or two.
Mirroring sketch elements, on the other hand, usually works pretty well, and I do it often.