JCB Petrol Chainsaw not cutting trees as good?

last week i cut 8 big trees down with my chainsaw with no problem, so today i was just going to cut the rest of the trees down and finish the job, so i noticed thechain was abit loose so i opened the thing up and tightened it, and i also added petrol and oil aswell, but the it doesnt cut through the wood properly it just makes a little cut and the cut itself goes all black due to the oil, it never did this before? and now i just cant use it... it takes ALLOT of effort just to make a little cut in the tree, where as before it was like putting a knife through butter (a bit of an exaduration..)