No, it's just going to take some time. There's an 80% chance that your Starmie will pass on its ability Illuminate, and a 20% chance that your Starmie will generate a Natural Cure variation offspring for every egg that is produced. Sometimes it's the first egg, and unfortunately sometimes it's the hundredth egg. Just keep on breeding and hope for the best.
If you're the kind of person that HATES probability factors and wants to know that you're eventually going to succeed, then you're playing the wrong game XD, but you can work toward an ability capsule....
P.S.
If you're breeding for something else too (like HP/IVs), then if you get a female Staryu with Natural Cure without the correct IVs, then you can choose to replace your Starmie with the Staryu to run an 80% chance of breeding a Natural Cure. This only works if the parent Ditto is running an IV breeding tool with all of the IVs you want on the offspring. If you don't know what this means then disregard it altogether.
