Sounds like it needs more water to me. Is the little fill tube hose actually hooked up to the "overflow" tube? (thats called a fill tube by the way). Is the flapper actually staying open until the tank is emptied? Is the flapper opening enuff to allow full flow thru the flush valve? Is there anything stuck in the flush valve? (thats the big hole the flapper covers). Some toilets just need a little more water and the fill valve can sometimes provide that extra little bit if you have one that fills quickly. In other words the new super quiet slow filling ones sometimes can cause this issue.
Also, if ya got little kids, they have a way of sticking stuff into toilets and causing all kinds of issues. 20 years ago before i was a plumber I had a problem with my toilet that had me stumped for days. I learned a lesson about what can get stuck in a toilet when i saw my toothbrush come sliding out the jet hole in the bottom of the bowl.