If this is a big problem, use the glass negative carriers in 11-C and use black strips of film or paper to block extraneous light from fogging the enlarging paper. Check the easel to see if the rubber strips on the bottom are even. Sometimes they are missing on one side. If so, prop the easel with thin strips of mat board to level it. Check the focus before printing each test strip and print.
Negatives can pop out of focus with the heat of the enlarger bulb.
If the test is not sharp, it is very difficult to make the proper judgment on exposure and contrast. Preheating the negative for 30-40 seconds might help reduce this problem. Try to keep your test strips flat in the easel so that you eliminate one source of non-alignment. It is difficult to assess contrast and exposure time adjustments if the test strip has moved during printing or is not flat to start with.