It depends. Sometimes the experiment isn’t designed very well and it means that not enough of the conditions are controlled so things aren’t happening in the way you think they are.
Sometimes there’s actually much more going on in an experiment than you think so it all comes out very weird.
And then sometimes you just make mistakes which change your results.
It’s always important to read up on what should be happening and to make sure you are doing a good experiment to the best of your abilities!
Analysing and interpreting results can always be quite tricky. Sometimes people expect a certain result and that makes them “see” that result, even if it’s not actually true. There are a lot of ways to avoid this type of bias, for example making sure the person doing the analysis doesn’t know what the different samples are (this would be a “blind study”)
Kathryn and Valerie have answered this question very well! Sometimes the result isnt clear and you have to go back to your data and process it / analyse it further but this can make you see the result you want. Controls are very important – what do you expect to see and do you see your “result” if you shouldn’t.