I was recently working on perfecting my bodice and making a fitted blouse out of a pretty blue linen. I put the sleeve in and magically it went in without a hitch!
When I went to try it on, well the fit was a bit off. After much head scratching I realized my error. Figures, the sleeve that would go in smoothly was actually upside down.
Bloopers always happen, no matter your skill level. But, for beginners it's always a good investment to stock your sewing library with great books and videos.
Try Sewing Machine Secrets by Nicole Vasbinder for learning the secrets to your sewing machine to get your started.