Depending on what sort of work was done with the dental..cleaning, extraction, etc it can take awhile for the pain of the work to ease up. When my Maxie had her dental she had the cleaning and 6 extractions. Even with the bupe for pain relief it still took her over a week before she was acting more like herself.
In my case the dental did not really make much difference with her glucose numbers,(over 1 1/2 months ago now) but it helped improve her appetite since she was much more comfortable when eating. Having a dental done is important, but it doesn't guarantee that the glucose levels will drop. Sometimes there are other issues at play or it will just take time for the pancreas to heal more,