In some cases, not all tweets will be displayed from certain Twitter users or handles. Twitter does not make available all tweets for search and it is worth noting that users who are less active or have incomplete bios are less likely to have their tweets indexed. Tweets from protected users will also be hidden from public searches.
If you do notice tweets missing from a specific handle, it is usually the case that they are also missing from the official Twitter Search. This means that Twitter have (for whatever reason) decided not to display certain tweets or handles from public searches. More information can be found here