There are a mumber of possibilities that this is being caused by number one on my list would be that your fkoi has become egg bound.
You really need to have this koi anethatized so that you can feel her under side .
Gently feel the swelling and if it feels but at the same to not too fluid then its certain your fish has un released eggs in her, normally a female spawns at a temperature of 20c, if the female doesnt spawn they would normally be re-absorbed
I would suggest you seperate this female off in a QRR vat with one or two other males and a few new mop heads or spawning mat if you have one at a temperature of around 18c then after a weej or so put the female into a seperate tank along wih a male then raise the tank temperature to over 20c, then sit down and let natur taje its course.if she does spawn all well and good if she doesnt she may have been uubduced into spawning condition which can been identified when the females vent is in a distended state ,
If she still wont spawn then I siggest you go to a koi deaaler and ask him for help in stripping her of her eggs [this is something you shouldnot try iy needs an expert] and if all this doesnt work you could try hormone injections however I'll be truthfull this can result in mortalities,
They are also expernsive and there is no garenteee that they will even work.
The experts suggest you then let her have a period of months in colder water over the winter period and then monitor the koi on a daily basis and if necessarry put her back in the anethetic water and let her drift of to a painless sleep .
See my article aon anethesia this will take you through the proceedure of its use with your koi when they need to be handled for infections etc :- .
https://www.gardenpondforum.com/articles/the-various-stages-of-anesthetics.5/
Be aware that there is a darker side to this as this may well be being caused by a tumour be a but only a vet with ultrasound equipment can tell you yes ir no and if it can even be operated on which may cost thousands of US Dollars ,
again the last course of action will be to monitor the fish daily then Euthanize when it starts to have major problems like with the swimbladder when the tumour starts pressing on it then Euthanize .
There are a bumber of anesthetics , we use one called MS222 but you can buy others Kusuri do a good one and they now sell in the US which is oil of cloves based at a dose rate of 10ml per 2 gallons US
I hope this information helps you with her and that you can resolve her reluctance to spawn as I dont really like the other option do you ?

If the