I have had this happen lately, I will win and then get failure. Also happens in Guild War, I will win then 97% (no screen shot).
This has been reported from time to time, unfortunately you will have to submit to Live Support and it is best if you have a screenshot or video clearly showing the issue.
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no use reporting it, i have done it many times and the respond always same as usual:
Thank you for contacting us.
We apologize for any inconvenience this has caused you. Regarding your concern, please be informed that after checking no anomalies found on the game on the reported date. Kindly try some of our troubleshooting methods listed below:
1. Uninstall Castle Clash
2. Restart Phone
3. Follow this link to reinstall the latest version of Castle Clash: http://bit.ly/2OJfbva
4. For Android 4.4 and up, switch the Runtime settings to Dalvik (Phone Settings > Developer Options > Select Runtime > Use Dalvik)
5. Switch between different connections (WiFi or 3G/4G) to test your connectivity with the game
6. Minimize the number of running background applications while playing Castle Clash
7. Minimize the number of devices connected to the WiFi while playing Castle Clash
Thank you for your support and have a nice day.
If you have any other questions,please feel free to contact us via Service Center or by email: Help.firstname.lastname@example.org.