Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
First, in light of Lady's history, this isn't unexpected. As I am sure you can appreciate, that anal gland was likely a bit sore and passing feces past it is likely uncomfortable at the moment. Furthermore, we don't tend to get alarmed with a 24 hour delay and I would be more suspicious that she is trying to hold it as opposed to being unable to pass feces. Still, we do want to ease this for her to make sure she can go and just avoid a secondary constipation in the next few days.
Now olive oil doesn't tend to be very helpful when dealing with the lower colon. This is because it tends to be digested before getting that far in the intestines. Instead, we'd want to use OTC oral GI lubricant. Examples of this would be cat hairball medication (ie. Catalax, Laxatone, etc), Miralax (1 tsp per 24 hours), lactulose (available OTC at your local chemist; More Info/Dose), or food grade liquid paraffin from the chemist. If she is eating, these can be mixed into her food. If you have to administer via syringe, do take care to avoid aspiration ( since that would cause problems we'd best avoid).
Furthermore, you don't need to withhold her normal food. The bran or porridge can be added to it. Other fiber bolusing options we do use in these cases are canned pumpkin or a 1/2 teaspoon of unflavored Metamucil/Benefiber. Just like people, these can restore fecal output regularity. In all cases, I would offer these with wet food to ease her eating of it, while making sure we are getting water into her (as canned food is 35% water). You also want to continue to encourage her to drink since this will help avoid hardening of the feces (which would make them harder to pass). Make sure she has have fresh water and you can even offer low sodium chicken broth if she won’t drink.
Overall, it sounds like Lady is holding her feces and trying to avoid going due to the discomfort she may have when doing so. If you felt she was very sore, then you can consider ringing her vet and requesting a short course of dog safe pain relief (ie Metacam, Rimadyl, etc). Otherwise, we'd just want to use the above to ease her passing of feces and getting things moving for her. Of course, while you are doing this, I would advise that you monitor fecal and urinary output. If you still have no feces within the next 12-24 hours, or she vomits, goes off her food, show belly pain, or straining, then we'd want a check with her vet +/- an enemas under sedation. Still based on her history, with the above, we'd not expect to need to go down that treatment route with Lady.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,