A common problem with a joiner until it is dialed in correctly is called snipe. Snipe occurs either at the start of the cut or at the end of the cut. Snipe is caused by not having the outfeed table height correct with the cutter-head. When joining most lumber, your depth of cut should be no more than 1/16" for most cuts. Two passes on two pieces of lumber to be joined and you cut off 1/4" of your expensive lumber.
There are YouTube videos to show how to set your joiner. The best way is with a dial indicator but you can also set it using a framing square. Once set, you do not move the outfeed table.
If you're joining long pieces of lumber, you may be allowing the board to to fall on the outfeed side.
If you don't have time to do it right the first time, when will you have time to do it over?