I had exactly the same routing (CAI-LHR...LHR-IST-LHR-CAI) ticketed by her in April. Obviously, Egypt and the UK are different countries. There must be some other reason.
I did have difficulty getting to the real reason for some of the initial objections. One of the problems I recall was convincing her that my second stop in LHR was not a stopover as it was less than 24 hours. She had the problem because it was an open ticket. I sent her a list of proposed flights showing it was not a stopover and she went ahead and issued the ticket open with the appropriate x/o indicators.
In answer to your question about reissue, yes, you can reissue and I expect you may be able to get the IST segment added back.
[This message has been edited by ExMo (edited Dec 03, 2003).]