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Excaliber is right, it will take a pretty good smack to the bottom of the mag to seat the mag when it is full unless you decide to push it in hard, which isn't really recommended. When you train a malfunction drill, you slap the mag you don't push on it to make sure it is seated. Getting off topic, but yes it is normal to have resistance putting in the mag underneath a closed slide. Just think of how hard those last few rounds are to get into the mag. It's that same tension you are pushing against to get the mag in. Glocks are the same and some instructors teach you to also download your AR-15 mags by a couple of rounds (depending on the mag) to avoid the same issue in the rifles. As has been stated (correctly) if the mag seats properly and easily with the slide locked back then you probably don't have a problem. Especially if you aren't getting malfunctions.