We've spoken before in these pages about the advisability (or otherwise) of downloading higher-capacity magazines by a round or two to increase firearm reliability and keep magazines in good condition. I'm a fan. I was taught in the South African military to download by plus-or-minus 10% for routine carry. Thus, 10% of a 35-round R4 magazine would be 3.5 rounds: therefore, it would be loaded with 32 rounds for routine everyday carry. The older R1 rifle (a license-built version of the FN FAL) had a 20-round magazine, which I usually carried with 18 rounds in it. In a combat area, where a fight was more likely, we'd download by only 5%, and some would load right up to capacity (although I never did, preferring to reduce the strain on the magazine spring and ease the cycling of the bolt). I've continued the practice in civilian life, downloading most higher-capacity magazines by 5-10% on a routine basis.
(Another common practice in some military units was to load tracer for the last 2 or 3 rounds in the magazine, to provide a visual warning to the shooter that it was almost empty and he needed to reload. I didn't do so, as the line showing the passage of a tracer bullet points right back to the shooter, and I didn't want to make myself an even more visible target, particularly at night. Also, in bush warfare in Africa, tracer bullets had the upsetting habit of setting fire to dry grass and leaves - not a good idea if the prevailing wind blew the resulting bushfire in your direction! In one incident, a fire started by a patrol during a firefight didn't bother them, but gained strength over the following day and intercepted their resupply truck that evening, destroying it and its cargo and forcing a premature end to their excursion. There was a certain amount of consternation and monkeyhouse over that, particularly from the infuriated truck crew . . . )
In answer to a reader's question about functioning problems with fully loaded 17-round SIG handgun magazines, a recent article discusses the downloading question more fully.
The top round in a loaded magazine contacts the bottom of the slide when the slide is forward and the magazine is fully inserted into the magazine well of a semi-automatic pistol. Although there is normally friction on the bottom of the slide from contact with the top cartridge in the magazine, it has minimal effect on the movement of the slide if the ammunition stack can compress slightly back into the magazine body against the magazine spring. If there is no room for the ammunition stack to compress, the friction resistance of the top cartridge in the seated magazine is too great to allow the slide to cycle properly, which causes the phenomenon that you are experiencing with the slide failing to fully cycle.
I would first suggest that you load your magazines to their capacity of 17 rounds, and then apply pressure to the top cartridge with your thumb. The cartridge should move against the magazine spring until it is visibly clear of all contact with the feed lips at the top of the magazine. This small space is necessary for the magazine to seat properly in the magazine well, and also for the gun to cycle properly and shoot reliably.
If you download your magazines to 16 rounds, I suspect your gun will exhibit the reliability you expect from it.
This phenomenon is not only limited to pistols, but should also be considered when loading and topping off any firearm utilizing a spring-loaded, box-type magazine. For example, it is a somewhat common practice to download an AR-15-style rifle magazine (particularly military-surplus magazines) from 30 rounds to 28 rounds to enhance reliability by ensuring the magazine will seat properly in the magazine well and the top rounds in the magazine will flow through the gun without causing any restriction or stoppage.
There's more at the link.
I believe the practice also extends your magazine's life, in that the spring is not kept in a fully compressed condition for long periods. I know many assert that modern magazine springs won't "take a set", as it used to be called, and be weakened by this; but I've had some older magazines that did, even from original equipment manufacturers. Third-party magazines. and some of Third World manufacture, may exhibit the problem more often. For example, I can't comment on their more recent production, but in my experience some older magazines from ProMag and KKK appeared to suffer from that issue. (YMMV, of course.)
Food for thought for those among us who rely on magazines to feed our firearms.