I don't have a good answer, without knowing more details. I've seen GUI discrepancies before, where it didn't calculate the number correctly based on what was in the blockchain. The best place to see if your rep score goes up or down is steemd.com. You won't see the calculated score (67) but you should see the raw blockchain number which should be easier to tell if it goes up or down. You shouldn't see cases where your rep score goes down from a post or comment that is downvoted, as long as the payout stays positive.
Note - if you start with X, then get upvoted worth 1000 points, then downvotes worth -800, you may see your score drop by 800, but it is still 200 higher than before the post. I.e. the net change is still an increase.
Thanks for your response @timcliff, I appreciate it!