Check Data

Check your (raw) data. Use lasvalidate to see if your input data is valid. If your input data is invalid, it is no surprise that the result is different than expected.

Check version

Check your program version. Do you use the latest version? If not, do so. Do you use the latest version and think this problem is new in this version: Try an older version!
It is may useful to run different versions in different directories. See the changelog for the latest version number. Use “LAScopy -h” to see your version number. Run all tools with the -v (verbose) option to see if they may generate a warning already.

Define your problem.

Extract it. Keep it as small and simple as possible. Break it down. By doing this, the problem may solve already

See if others had this problem

Use the search function with different keywords about your problem. It is most likely that someone else had the same problem before.

Post your problem.

If you got this far and did not find a solution, it may be worth posting your problem to the LAStools user group. Again: Concentrate on the issue. Again: Break it down. Supply some simple sample data so everyone can reproduce your problem. List exactly the commands you did run. What did they output? What did you expect them to output? Report the environment and version number you use. This is an example of a good question.