Synthetic construct with multiple ORFs not expressing
I'm afraid there's not much of answer but back to the bench with you! (That's why it's called re-search).

There are several factors that could be playing a role here.  You could have transcriptional or translational read through.   You should check your sequence and the cells you are going into to make sure you aren't getting Amber (UAG) suppression.

Before you do anything, you should check your DNA of all your groups.  Run it uncut on a gel and make sure the majority of the DNA is super coil.  The next easiest thing to check is translation problems.  Get an GFP antibody, possibly a few that recognize different epitopes, and see if your GFP is coming out the right size.  It may not be processed correctly or folded correctly. 

You can run into problems by optimizing the sequence.  First, synonymous mutations can cause folding variants (1).  Second, you might be causing a hair pen or other disadvantageous secondary or tertiary structure.  If you can't detect any problems in your Western, you might consider doing a Northern to trouble shoot the transcription.

If you rule everything else out, and you are just getting poor expression, then the only way I know to boost over CMV is to use T7.  The problem with T7 is always going to be what use for a source.  If you can use BSR/T7 cells and you can get your hands on them, they work great.  Otherwise you're looking at a T7 plasmid or MVA-T7 as common sources.  The plasmids never lead to a great deal of expression in anyone's hands I've ever known.  MVA-T7 works, but to say that it confounds things would be the under statement of the year.