Hi,
I had chronic constipation problems with my 6 mth old when I started her on weaning, and even now at 7 months she is still prone to it unless I'm particularly careful.
I tried prunes, stewed pears, prune juice (didn't like), diluted orange juice (didn't like), lactulose, plus stopped all cereal, unripe bananas and baby food jars (ones that contain apple, which is most). The only thing I found that helped was a combination of bicycle legs for a few mins several times a day, and undiluted apple juice which draws water back into the stool to soften it.
Even now, if DD doesn't have a dirty nappy for a day, I will offer her juice with meals and water between. If however, she is straining throughout the day with little result or hard-ish poo, I let her have apple juice in between meals as well until things are back to normal. This usually takes less than a day. She drinks it from a juice box with straw or from a cup. When she's not constipated, it's only water which she also loves.
HTH.
x