I have had problems with constipation for many years. Sometimes, miralax works. Usually fibre does NOT work that well for me, although All-Bran has worked well at times. I have not found any one thing that works consistently. Sometimes I'm just fine for 6 months and then I'll have problems for a few months. I've never been able to relate it to anything specific (stress, etc.) even accounting for maybe a time delay.
I would suggest trying all-bran or maybe miralax. The latter is one of the safest laxatives to take. You may have to take it for 3 or so days before it works.
Of course, check with your doctor.