A grinding wheel needs to be dressed or re-dressed at intervals. It's usually done with either a diamond tool or alternatively you can buy a handheld tool which has hardened steel wheels (only a few Euro). The tool is held against the toolrest and pressed against the face of the grinding wheel. This restores the straight profile.
Selecting the correct grade of abrasive is also important. In order to get a good finish you need fresh abrasive particles to be exposed quickly, otherwise the wheel becomes clogged. This means that if you want to grind hard materials you need a soft/weak grinding wheel and vice versa.
