This is a broad topic, but I'll try to address several possibilities. One thing to keep in mind is that when you start a pattern or mirror feature, "Features and Faces" (red arrow below) will be selected by default, and the first box (Features to Pattern) will be active. See the screenshot below. You'll see why that's important further down when I discuss specific situations. Also, for some reason unknown to me, the number of instances (third box down from the top, with 2 in it) includes the seed that's being patterned, so in the example below one new feature would be created. The numeral 1 is there by default on some occasions. Again, I don't know why, since the feature won't work with 1 entered there.
- If, for example, you're trying to pattern a single hole that was created as part of a pattern, and the software is trying to pattern all the instances instead of just the one you selected, that's because by default the first box under "Features to Pattern" was active. Clear that box and click in the one below it instead, then select the face of the hole. If it's a rectangular hole you'll need to select all four faces (or five if it doesn't go all the way through the body), etc.
- Keep the "Geometry pattern" box in mind. When it's not selected then the pattern will use the same conditions as the seed. For example, if you're patterning a hole, and the hole was created with "Offset from Surface" as the end condition, then the patterned holes will maintain the same offset from the same selected face.
If that box is checked then the same exact geometry of the seed feature will be patterned, ignoring the end condition.
- If you're working with a multi-body Part and trying to pattern a single body or combination of bodies, you'll get better results if you de-select the "Features and Faces" box and select Bodies 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 with Bodies. 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 had better results with creating a single feature or body with the sketch, then patterning 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.
If none of these address your issue, 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?).