I agree with everything munkynuts has said.
IME lactulose on its own is not appropriate or sufficient for genuine witholding associated with blockage and overflow.
How long has the witholding been going on?
When did you start the lactulose?
How much are you using?
Are you still using it?
Just to give you an idea, when I was dealing with a very similar situation, I didn't carry on the treatment long enough the first time it was prescribed because i just didn't grasp the severity of the situation.
My GP stressed that I would need to persevere for at least as long as the problem had existed amd probably longer.
She prescribed:
Docusate 10 ml daily
Senna syrup 10 ml daily
Lactulose up to 20 ml daily
Glycerine suppositories, one daily.
(Nowadays I think movicol is more frequently prescribed).
It took 5 months of this before I could stop the suppositories, and a further 6 months of the medication at full dose, which then had to be reduced gradually over another 3 months.
During this time, dd wouldn't poo in the potty or toilet, only in a nappy, and it took time and patience to get her over her fear of the potty/toilet.
For the next 2 years I had to be vigilant and watch dd's diet and fluid intake, and very occasionally had to revert to the suppositories to get things going again if she didn't "go" for a couple of days.
This isn't a quick fix situation, IME.
HTH