50% samples for same requests failing while execution of Jmeter script

I am executing Jmeter script and 50% samples for same request are failing. I am not sure why this is happening. Can someone help me to understand if this due to our server issue or because of my Jmeter script. Edited: I found out that some samples are not able to extract regex from previous sample due to which failure is occurred. Now I wanted to know why some samples can extract regex and some cannot?

Solution

Thanks everyone for you help. Issue is resolved when I updated user-agent in header manager for that particular request.

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