If it doesn't go on, it is because the friction on the tip is too high. That happens if the film is either not completely dry, or if it has an up- or severe down-curl. The best case is for the bend in the film tip to follow the curve of the reel. For that reason, it is sometimes better to retract the film into the cassette for a while rather than spool it straight after taking it out of the camera; the reason being that many cameras reverse the direction from that in which it is in the spool or on the reel. Some time in the cassette allows the natural curve to return.
Originally Posted by hdeyong
Don't cut the corners. Leave them perfectly flush. If you bevel them, the film is much more likely to climb out of the groove, and will cut into the layer above and scratch it. Film should load easily if it is flush at the tip.