If you are feeding carbs maybe avoid them for a bit, like melty sticks, bread etc
Lactulose is a really rubbish laxative in my experience. You could try asking the doctor to prescribe laxido? We used that for our little one when he’s constipated and it works every time.
Lactulose never helped us, so we are on movicol, which works wonders for us! Try warm bath as well xx
Have you tried a probiotic? We give one to my LB and it helps to keep him regular. I did some research and chose Optibac, but there are others aswell x