1/ Too coarse a crush on the grains. Probably number 1 culprit.
2/ Volumes off - mash, sparge, preboil. Be sure to measure mash and sparge water accurately, account for grain absorption and dead space, and get an accurate boil off rate/hour on your kettle. This will help you hit your final volume into the fermenter accurately. If volumes are high or low, it affects your concentration of sugars and therefore your OG measurements.
3/ Temp accuracy - you need a thermometer that's accurate enough to hit your mash temp fairly accurately. If you unknowingly are mashing too high or too low, you could be outside the optimal enzymatic range for good conversion.