Slow bud growth

The leaves look nice really good with a slight nute burn at the tip but other then that they look very healthy .

Zkittles autos were not my favorite grow. Extremely sensitive to N, K, P, wind burn light burn, and pH. Some were done in 9 weeks others 16 weeks. To be honest yours don't look bad at 52 days considering it probably took 3-4 weeks before flowers started to form. I would say have patience. They look healthy and may be on the longer life cycle.

Yep, they look fine, if you flipped to 12/12: 52 days ago. the first week of flower usually doesnt start for 1-2 weeks after flip. Which means most likely yhat youre in the 4th-5th week of flower. judging by the height, its likely a sativa dominant pheno, which flowers slower than indicas (generally) Dont worry too much about it, the plant will finish when it finishes, harvest based on your best judgement (bud /trichome development) and dont harvest based on age Best of luck with the rest of your grow!

Maybe give them a good dose of flower nutrients with some bud swell or molasses compost tea nutrients

Heck could even fall under co2 deprivation too. Just things to consider researching.

The other thing to consider would be ph as a nutrient lock out could stunt certain development. But I do feel Hashy is right on this one.

They look like I would expect for the age. Nothing to worry about. Good luck