In some locations, Combee only spawns in flower patches.
The flowers on the ground have encounter tables separate from the main overworld encounters. While encounter and sparkling power do affect these spawns, it may take a couple tries of going back and forth to get an encounter from a flower patch. Since such a limited amount of Pokemon spawn from flower patches at once, this means it will take a while of going back and forth before you find a shiny, and the shiny is likely to be a phase of another flower patch Pokemon or something outside of the patch entirely. Although flower patches have some unique hunts (like Oricorio Pom-Pom, Sunkern, Spewpa, and Dolliv, the latter two of which I haven't yet gotten from the patches), there are better places to hunt Combee that are far easier hunts.
South Province Area 2 is the best place to hunt Combee. Combee can spawn all over the fields near Cortondo, and with Encounter Power Bug 3, the only competition you have is Kricketot. If you can't stand the competition, you can reset your outbreaks until you get a decently placed Combee outbreak.
Alternative, there are a bunch of flower patches in North Province Area Three around the Team Star fairy base. While an encounter power bug sandwich can net you a group of four Combee and a Vespiquen, the competition from Vivillon is too much, so if you would rather hunt for Vespiquen instead, resetting to get an outbreak here is your best option.
This is a rather minor point, but I'm not sure if you can get functioning Combee outbreaks from singular flower patches. I've gotten exactly one while playing the base version of the game that didn't spawn anything, but I don't know if it was because I was playing the base version or if it was because the outbreak spot just didn't work. Either way, outbreaks near Cortondo are more ideal.
I hope this helps and you're able to get your shiny quickly!
