Originally published in Strategy & Tactics #57, this game on the Smolensk campaign of 1941 had a bunch of new and exciting features, such as untried Soviet units and German motorized units with divisional integrity bonuses. It became very popular, and was followed by a long string of similar games...only they weren't, quite. It seemed that just about every game that claimed to be "based on the Panzergruppe Guderian system" had some tweak or change that made it sufficiently different to require a thorough perusal of the rules to discover how untried units worked, what the requirements were for divisional integrity bonuses, etc. By the time Operation Grenade appeared five years later in S&T 84, any resemblance between the new game and its distinguished ancestor could only be found by rigorous rules exegesis or generous application of handwavium. Yes, both games dealt with combat at the operational level, had untried units, and divisional integrity rules, but anyone trying to play the latter by assuming its rules were basically the same as PGG would have been in big trouble.
By the same token, when Fifth Corps appeared in S&T 82, it was supposed to be the first of five games in the "Central Front Series", an ambitious attempt to create a simulation of NATO/Warsaw Pact combat in Germany st the battalion/regiment level. Again, innovative game mechanics were employed and the grognards went nuts, sending the game's approval rating sky-high. It took almost ten years and two changes of ownership for the final game in the series, Donau Front, to see print in S&T 131...but by then the rules and graphics had been dumbed down to the point where you couldn't integrate Donau Front or its predecessor, North German Plain, with the first three games of the series. In fact, Greg Costikyan's comprehensive list of SPI games doesn't even list the latter two games as part of the series, and the "standard rules" for the system supplied with Donau Front admit that the unit strength calculations and reorganizations make linking the games impossible.
Okay, so there are prefectly good reasons why the Central Front Series fell apart, but what doesn't make sense is the whole ongoing process of messing with something that worked well, to the point where nominally similar games using what were alleged to be the same fundamental designs looked and played nothing like each other. Going back to the PGG system for a moment, it boggles the mind to realize that the system was applied not only to the Army Group South Quad (and its "fifth wheel", Kharkov), but to such wildly dissimilar campaigns as the 1942 Stalingrad offensive and Soviet counterattack, the hypothetical Invasion America, the aforementioned Victory in the West Quad (which had two spare wheels, Sicily and Operation Grenade) and probably the most extreme example of the system being mutated beyond all recognition, Eric Goldberg's Kursk. (My take on Kursk is that it actually simulates the battle rather well - it's an unplayable simulation of an unwinnable game, at least if you are playing the Germans.) I think this constant process of tinkering with the rules may have contributed to the decline of the hobby, since it became just too damn hard to remember the rules for all the different games. It was bad enough that the hobby demanded so much time to set up and play the games in the first place, but the constant modification of the basic rules systems made it nearly impossible to introduce new fans to the hobby with anything current.
Just another reason why the hobby collapsed, I guess.