Odd. When the slide is locked back you should feel almost no resistance. You'll feel a tiny bit about halfway in as the mag contacts the mag release and drags along the side. You may feel just a tad more the last 1/8" or so as the follower starts to push up the slide lock (assuming the mag is empty) and then the mag release snaps in.

Do the mags drop free when you release them or do you need to tug on them a bit?