Amazon will regularly rotate the Buy Box from one seller to another without any price change taking place. One of Amazon's limitations is that they will not send us a price change notification to let us know if you won/lost the Buy Box, so we will only find out after the next competitor price change.
Example: if you win the Buy Box and you have selected the scenario 'If Buy Box Winner, Do not reprice', then your SKU will stop repricing. If you lose the Buy Box when Amazon rotates it to another seller, RepricerExpress won't find out, will still show you as Buy Box Winner and will still follow 'Do not reprice' until a new price change occurs, then will try to compete for the Buy Box again as we're now aware that you lost it.
To help with the Buy Box rotation, we would normally recommend to choose 'Reprice as normal' for the scenario 'If Buy Box Winner'.
Please note: if you are using the option 'Price only against the seller who holds the Buy Box spot', it wouldn't be possible for you to choose it.