You might also look for information on collimation. You may have misalignment of the primary or secondary, or more likely, both. This will disallow sharp focusing.
If you question, for some odd reason, the main mirror or lens, you can remove it and use it with a candle or other object and do what was mentioned above. At some point you will get a nice focus plane where the image should be sharp.
Try not to remove the objective unless you have to. If you do, a good refractor has two lenses, one to correct for chromatic aberration of the other (this is called a doublet). They usually put a pencil mark across both lenses so they can be put back with the correct orientation with one another. Pay attention to that.