Repeat Region Relations work in one drawing table but not the other

I'm no repeat region relations expert, but are those two parameters present at the bottom of the relations window?  If not, add them and see if that does it.

I remember trying to add a relation that substituted A/R for the qty on bulk items and I had to manually add parameters in the relations window, even repeat region parameter like rpt_qty. See this thread:

Bulk Item Qty in Repeat Region

Did you try to reboot and the problem go away? 

Seriously, I tried to to replicate your problem and managed to get weird results that might be a clue as to how to solve your issue:

I don't have a file set with your parameters, but I definitely got into some trouble when I deleted a parameter that was used in this repeat region relation:

if exists("ASM_MBR_FILE_TYPE")
  TYPE_STRING = "** Present"
else
  TYPE_STRING = "Missing"
endif

(the drawing has a repeat-region table with a column that has the entry "&rpt.rel.TYPE_STRING"; which will either show "** Present" if the component listed in the particular row has the parameter "FILE_TYPE", or it will show "Missing" if it doesn't - note my test assembly had a mix of both types of parts).

When I intentionally deleted the ASM_MBR_FILE_TYPE parameter in the repeat region relation ui's local parameter list, and then regenerated the drawing, the relation no longer worked (as expected - every row was filled with "Missing").

When I then tried to recreate the parameter ASM_MBR_FILE_TYPE, either by temporarily creating the extra table column which showed the component parameter "FILE_TYPE", or simply by manually adding a Real Number parameter "ASM_MBR_FILE_TYPE" to the local parameter list, the the relation no longer worked (all rows were now showing "** Present".  Also note that the parameter ASM_MBR_FILE_TYPE did change to the "Unknown" type.

But what actually worked, incredibly,  was simply going into the actual text of the relations (lines 01-05 above) and making a non-change - for example, retyping the word "endif" on line 05.  After that, the relation prescribed in the very same looking 5 lines actually worked...

Maybe the same thing is happening to you in some way - try making a similar non-change in your relations of the repeat region in the new drawing and see if that helps...  I doubt it because I don't actually get the message "There are errors regenerating repeat region relations", but who knows???

Try the following code and make sure you've got &rpt.rel.RELSHOWNAME in one of the repeat region table fields.

Untested:

IF EXISTS("ASM_MBR_D_UB_DWG") & EXISTS("ASM_MBR_D_LB_DWG")
 DESCDUMMY=ASM_MBR_DESC
ELSE
 DESCDUMMY="N/A"
ENDIF
IF EXISTS("ASM_MBR_DESC")
 IF DESCDUMMY=="FLEX BODY"
  RELSHOWNAME=ASM_MBR_D_UB_DWG
 ELSE
 IF DESCDUMMY=="NON-FLEX BODY"
  RELSHOWNAME=ASM_MBR_D_LB_DWG
 ELSE
  RELSHOWNAME=ASM_MBR_NAME
 ENDIF
 ENDIF
ENDIF

Doug Schaefer‌ is correct, the parameters referenced in your relations need to be listed in the parameters section.  (Remember this Doug: Re: Bulk Item Qty in Repeat Region)

The required parameters will automatically be added to the repeat region parameters section  if you add them to one of the repeat region columns (like MartinHanak‌ mentioned), or you can just manually add them to the repeat region parameters section yourself.  They do NOT need to be in a (hidden) column of the table.

Finally, "IF EXISTS" does work in repeat region relations (and this is the only place it works).  See this discussion for a sample: Relations in drawing BOM

Let me know if you need screenshots demonstrating all of this. 

Good to see so many people jumping in to answer a repeat region question... and great content everyone.

Has anyone else than me never used IF EXISTS for repeat region relations? In my experience if that parameter doesn't exist the relation simply does nothing.