I have gone through your query and would try to help you in the best possible way.

Possible causes:

(1) Resistance:
There comes a time for most of the drugs body shows resistance to them and there effect gets lessened, this can be compensated by a higher dose.
(2) Opioids:
After such a surgery surgeons use pain killers and most of them ,used have Opioids in them and they have a very common side effect' they worsen constipation
(3) Sleeping pills,anxiolytics:
Some surgeon prescribe anxiolytics or sometimes anxious patients start them ofn their own and these also have an adverse effect of worsening constipation
(4)Diet changes:
Patient is reluctant to eat well and automaticly gets inclined to liquids so less stool volume is formed and relative constipation occurs
(5) Life style changes:
After surgery Patient gets less active , so his systems including digestive system alters its functon.

SUGGESTIONS: In my opinion

(1) Start walk regularly if you can.
(2) Eat fiber,porridge ,brean bread,and other high fiber meals,
(3) Drink more water
(4) You may consider changing laxative like Organic laxative tea, Lactulose, Lactitole monohydrate (proved to be very effective and sodium nitropruside (though it has a reputation of abdominal cramps.
Discuss these causes and options with your doctor if you feel like and hopefully you will see results

Hope this answers your question. If you have additional questions or follow
up questions then please do not hesitate in writing to us.
Wishing you good health