I put mine in left to right without problem. It allowed me to move the stock rear halfway off the spring and slide the new sight right over it, driving the old off with the new, and never having to mess with the spring. I don't think that's the problem.
Mine slipped in so easy my toddler could have installed it. I guess they have modified their design somewhat due to some criticims and fears of it falling out, or your slide notch is slightly smaller. I bet on the former.
In my experience the best way to fit a to tight sight is to slowly remove metal from the bottom of the sight. Use a stone or fine file, lay it on the table, place the sight in top of it, and move it back and forth making sure you remove metal evenly. It doesn't take much, so go easy and keep on checking.
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