Ligating large fragment into vector

[jimmy barbee]

I have been having trouble ligating a 4 Kb PCR fragment into a 4 Kb 
vector.  Whether into a T/A vector from the PCR reaction or after cutting 
to leave sticky ends into another vector, I haven't been able to get a 
single insertion.  Can anyone suggest modified ligation reaction 
conditions to help?  I suspect a site on one of the ends isn't cutting 
well or at all, but I don't know why the T/A approach didn't work.

Thanks,

Jimmy