@groodt
Thanks for providing the list of ISINs. I checked a sample of ISINs in the file you sent. All the ISINs I checked refer to inactive issues: matured or called bonds, or bonds in default.
Taking the title example of US001765AC09 that you gave, you can see in the screenshot of the bond view you provided that this bond is in default. In the same screenshot you can see that the chart is drawn from the RIC 001765AC0=RRPS, which provides Thomson Reuters evaluated price rather than from the RIC US001765AC09=BGCP, which provides contributor pricing from BGC Partners. If you use RIC 001765AC0=RRPS, you can get yield history up to 28-May-2014 as in the chart. Here's the worksheet function you can use in Excel: =TR("001765AC0=RRPS","TR.YIELDTOMATURITY","Frq=D SDate=2014-05-28 EDate=2014-04-28 RH:Date").
I hope this somewhat explains the issue you experienced. The question remains why when you use ISIN in your call, the system translates it to the RIC US001765AC09=BGCP instead of 001765AC0=RRPS. In other words you can ask why =TR("001765AC0=RRPS","TR.YIELDTOMATURITY","Frq=D SDate=2014-05-28 EDate=2014-04-28 RH:Date") returns yield history while =TR("US001765AC09","TR.YIELDTOMATURITY","Frq=D SDate=2014-05-28 EDate=2014-04-28 RH:Date") returns error "[Handler_BackEndFailure] Metadata request for US001765AC09=BGCP failed (no such RIC)". Since the issue is easily reproduced using Eikon GUI and Excel, I suggest you raise it to Thomson Reuters support by either calling Thomson Reuters Helpdesk number in your country or by using Contact Us capability in your Eikon application.

Hi @groodt,

I was able to see the same result as you. I will bring it to the attention of the Eikon Data APIs team for further investigation.

thanks.

Thanks!

Attached a list of ISINs, where the error has occurred. ~ 3500

Maybe this is helpful for further investigation.

isin-witherror.txt

I have not double checked for all of these ISINs if data is available, but Tracing cusip was possible without a problem, therefore I assume so.