At first ensure that the competent cells you used for cloning has high competency at least 10^8, the other thing u can try is giving polynucleotide kinase treatment to your inset and setting up ligation reaction simultaneously in 1:3 , 1:5 ratios it may help.u can also try using quick ligase as an alternative.

I would suggest using electroporation instead of chemically competent cells. Plasmid size has a pretty strong negative effect on efficiency of transformation, but much less so with electroporation. 

6.5 KB plasmid is not preety big for a plasmid , i have been sucessfull with plasmids as large as 8 kb.Also electroporation is a subject to availablity of the instrument your institute.One important point i want to highlite is -1) do u use positive control i.e native ,undigested vector as transformation ctrl. (2) do u use negatice ctrl, double digested vector ....if u find colonies on your positive ctrl the problem may be with your ligation.

all d best .

Heat inactivate the ligation mixture before transformation, this should improve the transformation efficiency many folds. Also, try using ultra competent cells for maintaining big plasmids, like XL-10 gold or XL-1 blue, both from Stratagene.