Welcome! My name isXXXXX am a 2003 graduate, and currently a Medical Director of a veterinary hospital.
I am sorry to hear about this concern for Kazon.
Okay. So, it doesn't sound like you are stating that there is any "straining" to urinate. Straining would be usually observed if there was any obstruction to urinating
like from the prostate hypertrophy (enlargement).
If there is no true obstruction, and the bladder doesn't seem to be expressing, that can suggest a different problem like bladder atony.
Now, you are not trying to treat urinary incontinence. You are trying to deal with a bladder that is not contracting. If that is the case, you should ask your vet if a trial on bethanecol.Bethanecol in Dogs
So, it may not be that muscles are not relaxing, but the opposite, where the bladder is not properly contracting. Not as common, but it can occur.
tough question, but best info I could throw out there. Definitely, if you have already worked with a specialist, make sure to get them involved as well.
I hope that information has been helpful.
Please remember to select Reply to Expert, if for any reason you need further clarification, have more questions, or were expecting a different type of answer. Prior to leaving a feedback rating, which I greatly appreciate, my goal is to provide you the best answer possible.