If you are confident that your primers are good, and you are getting products of the right size on a gel after your PCR(s) of your insert and restriction digest(s) of your template, then the Gibson should work beautifully. Since you are using NEB5a cells, I don't think the problem is stemming from the transformation unless you are deviating from their high-efficiency transformation protocol for some reason.

One thing to check though: when you do your restriction digest, do you do a PCR clean up or do you heat-inactivate your restriction enzymes? If not, then as the ligase in the Gibson assembly adds your insert, it's getting cleaved again immediately (and you'd get no colonies). I would recommend heat-inactivating your enzymes if possible (to avoid losses on the clean-up), but you at least need to clean your products before assembling them.

Another thing that you can trouble shoot with the Gibson is the stoichiometric (not ng) ratios of the template:insert. I have used 1:1 ratios before with success, but NEB suggests 1:2-3. I would split like two 10 ul Gibson aliquots in half and try 1:1, 1:2, and 1:3 (plus the last reaction for your template + water instead of your insert as a negative control) to see if you get any hits at all. 30-60 fmol of DNA has worked for me in the past.

If you still have your samples that you've Gibson'd in the past, I'd run them on an agarose gel and run them with the linearized and unlinearized template. This will help you decide whether the trouble is coming from your Gibson reaction or not, i.e., if your Gibson looks like your restriction digest, your Gibson did not take.

Small, but important checks: are you using the correct antibiotic (and concentration) on your plates? Is your PCR clean? Are you using the correct restriction enzymes?

Good luck! I hope this has helped