Your gravity reading is only part of the equation. What was your volume versus what the expected volume was supposed to be?
Then add up the gravity points:
(Target OG-1)*1000 * target volume = expected gravity points
(Actual OG-1)*1000 * actual volume = actual gravity points
If the numbers are pretty close then the issue is a volume issue and you need to look at your equipment profile
-- you are extracting the correct amount of sugars based upon your projected efficiency
-- your water volumes are too high
If the actual gravity points is greater than the expected points, then your equipment profile is off and you had a better conversion/extraction efficiency than you had planned on
-- your water volumes are too high
-- you need to adjust your profile for a higher efficiency
If the expected gravity points is greater than the expected points, then you had poor conversion/extraction efficiency
-- there are a number of issues that lead to low efficiency. So breaking it down further:
* if your water volumes are close (predicted vs actual):
-- look at the crush of the grain, it may not be fine enough
-- look at your mashing time, you may not have enough time for extraction of the sugars once converted
-- you may not have fully dispersed the grains into your mash water, leading to dough balls
-- your sparge was poorly performed -- not enough mixing, contact time, channeling of the sparge water, etc.
-- you will need to adjust your recipe efficiency to reflect your actual results
* if your actual water volume is much greater than your predicted volume:
-- the above list is still applicable, but you also need to look at your equipment profile and readjust the water volumes
* if your predicted volume is much greater than your actual volume:
-- you really have a mess. Start by adjusting your equipment profile for volumes and actual efficiency
-- follow list above for troubleshooting your process