On some digital VOM's, you have to switch the positive lead to another socket to read Ohms. I always forget to do this. This socket may say ohms or just have the omega symbol under it. Also, on some of the conventional "needle" (not digital) type meters, you have an "ohm aXjust" thumb wheel that is used to zero the needle when the two leads are held together. If this wheel is way out of adjustment, you wouldn't get a reading when calibrating the meter.