The optical drive slot in the bottom case is probably deformed (known issue) causing a gap where the locking clip is located. Take off the top case and with the flat side of a spudger inserted in the middle of the slot slowly do a rotating movement to gradually lift up the upper section of the aluminum slot. Apply a ruler on the front top of the bottom case, pry the metal until it reach the ruler. Take your time and go slowly. If you don't have a spudger use a flat screwdriver with electrical tape around the tip so you will not damage the aluminum. Don't insert the flat tool inside the optical drive but only inside the aluminum slot.
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Once the deformation is corrected reinstall the top case and before fastening the clip located rear of the optical drive slot insert something in the slot that will prevent the slot to deform when applying pressure to snap the clip.
The optical drive slot in the bottom case is probably deformed (known issue) causing a gap where the locking clip is located. Take off the top case and with the flat side of a spudger inserted in the middle of the slot slowly do a rotating movement to gradually lift up the upper section of the aluminum slot. Apply a ruler on the front top of the bottom case, pry the metal until it reach the ruler. Take your time and go slowly. If you don't have a spudger use a flat screwdriver with electrical tape around the tip so you will not damage the aluminum. Don't insert the flat tool inside the optical drive but only inside the aluminum slot.