I was working in the reminders app last night and noticed that it failed to sync across all my devices, which run the latest version of OS X and iOS. I've tried to sign out of iCloud on my iOS devices but it keeps asking whether I want to delete the content from my iPhone and wasn't sure what to do.
Try turning reminders in icloud off and back on. It might prompt for keeping the existing reminders locally when it is turned off in which case you can select
yes. When you turn it back on, it might ask for merging reminders and select
yes again. This has worked for me in the past.