As others have sort of referenced, Muster faced a number of difficult draws at the French. This was partially due to only 16 players being seeded back then. The 32 seed format didn't come into play until several years after Muster had left the tour. Certainly the luck of the draw is what it is, and we'll never know if anything would have gone differently for Muster, but he absolutely did not get much luck.
He was forced to play Courier twice in the early rounds in the midst of Courier's prime in 1992 and 1993. In 1994, he drew Agassi in the second round, beat him in a five set thriller, and was rewarded with a 3rd round matchup with Rafter, who posed a terrible matchup for Muster on any surface. Muster's draw looked pretty good in 1996, but the dangerous, experienced Stich, who was no stranger to Grand Slam success, played with nothing to lose and frustrated Muster into a poor 4th round performance. Stich went on to nearly win the tournament.
As another poster mentioned, 1997 was an oddly down year for Muster on clay. He started the year on fire on hard courts only to bizarrely lose early on clay week after week in the spring. Muster partially blamed his struggles on his transition to a new extra long racquet, which helped him on faster surfaces, but wasn't what he was used to on clay. I'm sure there was some truth to that, but I always felt that Muster was slightly fatigued from all of the hard court matches he played from January through March and maybe even slightly less hungry for the clay season, having accomplished a majorly satisfying victory in Key Biscayne. Regardless, most felt that he could still contend at Roland Garros. He got through his first two matches without playing that well before having the misfortune of drawing then unheard of Gustavo Kuerten in the 3rd round. Muster played very well for periods of that match and actually led 3-0 in the fifth set, but Kuerten's serve and ability to go from defense to offense with huge angled groundstrokes from well behind the baseline were too much. Kuerten famously went on to win the title, beating Kafelnikov, Medvedev and Bruguera among others. Ironically, those were all players that Muster typically dominated in his career. Despite his less than stellar form that year, had Muster found a way to get past Kuerten, he would've become a big favorite to win the title.
Looking back on things, I believe Muster set out in 1998 to make one last big push on clay. He didn't play well in Australia or in Dubai and had only played three official tour matches until making a strong run to the semis of Indian Wells in March. Muster skipped Key Biscayne and instead got ready for the clay court season, where he performed much better than in 1997. Though it was nowhere near the vintage Muster of 1995-96, he did reach the final of Estoril, along with the quarters of Hamburg and St. Polten and the 3rd round of Rome. Ranked in the 20s and thus unseeded at the French, Muster actually received a "favorable" draw in 7th seeded Jonas Bjorkman. Muster easily beat Bjorkman, crushed German up and comer Nicolas Kiefer and then got through four and five setters in rounds 3 and 4 to reach the quarterfinals. Spanish clay courter Felix Mantilla, who Muster had beaten in a testy match in Rome a few weeks earlier, took Muster out in four sets, effectively ending the Austrian's last decent Roland Garros hope. In 1999, Muster seemed to have quietly decided to play through the clay court season and then fade away into retirement without telling anyone. He recorded a good result in Sydney to begin the year, but that was about it. He disappeared after a first round loss to Lapentti at the French.