Luke's Gospel says Jesus healed the severed ear of the slave of the high priest (Luke 22:50-51) but the other gospels don’t mention it. Surely a miracle such as that would have astounded the people arresting Jesus! Why did only Luke include this detail?
Regarding the differences in the accounts of the slave in the garden, it's common for the Gospel writers to highlight different aspects of a single moment, depending on their particular emphasis or memory, etc. Luke was not present in that moment, so he relied on Paul's testimony (which Paul gained directly from Jesus according to Galatians 1), while Mark captured the perspective of Peter, who was present, as was Matthew and John.
The slight differences in each account are an inevitable consequence of multiple eyewitness testimony, and those differences serve to reinforce the trustworthiness of the Gospels (rather than detracting from it). We possess four Gospel accounts rather than only one because of these differences. The Lord used the different perspectives of these four men to produce a complete picture of each moment recorded in the texts, therefore students of the Bible must integrate the four views into a single, cohesive story, yet never in such a way that one Gospel contradicts another (they don't).