Cloning Advice?

Over the past few weeks I still have been struggling with successfully cloning my SUMO mutant into a fluorescent protein.  Every time I try I have successful digests that come out nicely on the gel.  Therefore, I think my issue is somewhere in the gel purification/ligation steps.  After the first digest I usually have anywhere between 50-90 ng/mL of DNA for each sample (vector and insert).  However, after the gel purification this drops to anywhere between 5-20 ng/mL.  I’m concerned that I do not have enough DNA going into the ligation for it work.  I have tried some ligations with an excessively high insert:vector ratio and even those have not worked.  Moving forward, I do not think it is an issue with my sub-cloning methods because once I sub-cloned some pure m-Cherry plasmid along with my ligations, to start a midi prep, and the plates with pure m-Cherry grew while the plates with the ligations did not.  I am considering cutting down my elution buffer at the end of the gel purification from 30 ul to 20 ul in order to increase the concentration of DNA going into the ligation.  Other than that, I feel like my only option is to keep trying and hope that eventually everything works.  I know that many people have had trouble with cloning at times, and I was wondering if anyone had any tips/suggestions of what worked for them?

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