The problem is the dxf file is incredibly 'busy', check out just this ONE hole:
Attachment 491108
notice there's NINETY-FOUR NODES!!! in JUST that one circle!!
the fix: You need to use Corel's "OBJECT SURROUND" button/feature...
NOTE, this button only shows up when one/some closed objects that CAN be re-outlined are grouped...
OPEN objects can be re-outlined IF you close them, by connecting disconnected nodes...
Attachment 491110
Anyway, this worked great for this dxf :)
BTW, it took my Corel nearly 20 seconds to re-outline those circles, and my Corel is FAST--
Here's a pictorial lesson ;)
Attachment 491109
And, here's the corel job all saved-- the RED outlines are ready to rock! :D
Attachment 491111