Hi there. I hope I can help you figure out what's going on with your girl.
First, I'd have her on a daily immune building diet that is appropriate for her age. The main diet will be free-choice laying feed, the feed you're offering. First question about that - is she eating the feed? Feel her weight - does she feel heavy or light to you? Does she feel heavier back between her legs and towards her vent? Feel very delicately there in case there's an egg. Does she feel full there? Hard? Mushy? Lumpy?
Check her very carefully for parasites. This will require one check during the day (at least) and at least three checks at night (for mites, which only come out, then only occassionally, and rarely in the daytime). Look very very carefully, every inch of feather, skin, etc - for the mites/lice themselves (they're nearly microscopic) and for eggs/nits of the lice around the feather shafts, the feathers themselves, around the vent, etc. Tipping is a sign of weakness, a vague symptom of nearly anything, but very common with mite infestations. If you see any bugs after these searches, let us know and I can tell you how to treat them thoroughly.
Also, she's nearly laying age. But you don't know if she's laying. While you're checking her for external parasites, look at her feathers. Do you see a lot of "pinfeathers", the new incoming feathers still in their casings? Do you see a lot of new feathers? Or no sign of them.
Also check her vent. Is it small and tight, like a tiny o, or is it dilated, moist, and open a bit more like this: _ ?
I am concerned that she's nearly of laying age but not passing a first egg. Does she seem to do a penquin walk at all? Squat? Lay down a bit? Hang around near the nest box? Look distracted? That weakness often accompanies the first eggs getting into the system. If you're using crushed oyster shell (which has a lot of powder, that I call 'flour') then mix that flour of the oyster shell into her feed every other day for 3 days. Also - give her polyvisol (for its vitamin D) every day for a week, 3 drops directly in the beak. Also please do give her yogurt daily until this is resolved - both for the live bacteria probiotics as well as for the slight vitamin D activity and added calcium. For the 'flour', just get the floury bits and mix them with your pellets/crumbles. You can even mist vegetable oil on the top portion of feed and mix it in there to help it stick.
I'd also feel her carefully for any feel of fullness that might be an egg that is unwilling to come out. This is especially if her vent is more dilated and wide, rather than tight and small.
You can even try to feel there for one inside. Put on a glove, lube it with KY jelly or with oil. Using the index finger, enter the top portion of the vent like you're going to carefully run your finger along the top wall of the inside of the cloaca. Using your other hand, gently very very gently push her abdoment lightly up to 'sandwich' any potential egg in the system. Palpate her to see if perhaps there's a very large egg there.
A word of caution - if you ever know there's a broken egg inside, it's imperitive to remove all of the egg from the cloaca. You can pull out whatever shell you can. Then use a baby's ear syringe (every chicken owner benefits from having one around) and *cool* (not warm) water to flush out the remaining egg. The cool shrinks and cools the cloacal tissue to reduce changes of prolapse. Warm makes us feel better but doesn't help the bird unfortunately.
If there's an egg too large to pass, you can press it to the vent opening and puncture it where it's end it poking out of the vent - collapse the egg, making an effort to get all the inside of the egg to the outside that you can. Pull the shell out and flush for a good while with cool water. You can also mix a little nolvasan (another wonderful product both for man and beast) in the flushing water for a light antiseptic effect, great in case any egg shells hurt her lining but without hurting her good bacteria in her vent mucous lining.
Ideally you want to feed her up with calcium and vitamin D in case she is not forming good egg shells. Her first egg shell should be nice and firm so that it will pass more smoothly through her cloaca. I'd seriously suspect that she's working up to laying.