The LED forward voltage drop will vary between manufacturing lots. Each time the LED manufacture produces an LED, the forward voltage drop can be slightly different from the last. Refer to the LED manufacture datasheets to determine the minimum and maximum possible forward voltage drops. Using these values along with your calculated resistor value, recalculate for circuit current draw. You may find that your current has drastically changed due to the voltage variation across the LED junction. The best solution is usually to reduce the total number of LEDs per series circuit. This will minimize total variations in forward voltage across the junctions. A resistor LED driven circuit may not be feasible in your application if a reduction in LED per series circuit is not possible, or is possible but total current remains unstable. In this case, you must revert to the recommended constant current source circuit configuration, as a substitute for the resistor LED driven circuit.