You were missing to set to true the IncludeDetailedScans element. Here is an example of a SOAP Envelope:
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body>
<TrackRequest xmlns="http://fedex.com/ws/track/v6">
<WebAuthenticationDetail>
<CspCredential>
<Key>CSP_KEY</Key>
<Password>CSP_PASSWORD</Password>
</CspCredential>
<UserCredential>
<Key>DEVELOPER_KEY</Key>
<Password>DEVELOPER_PASSWORD</Password>
</UserCredential>
</WebAuthenticationDetail>
<ClientDetail>
<AccountNumber>ACCOUNT_NUMBER</AccountNumber>
<MeterNumber>METER_NUMBER</MeterNumber>
<ClientProductId>CLIENT_PRODUCT_ID</ClientProductId>
<ClientProductVersion>CLIENT_PRODUCT_VERSION</ClientProductVersion>
</ClientDetail>
<Version>
<ServiceId>trck</ServiceId>
<Major>6</Major>
<Intermediate>0</Intermediate>
<Minor>0</Minor>
</Version>
<PackageIdentifier>
<Value>TRACKING_NUMBER</Value>
<Type>TRACKING_NUMBER_OR_DOORTAG</Type>
</PackageIdentifier>
<IncludeDetailedScans>true</IncludeDetailedScans>
</TrackRequest>
</soap:Body>
</soap:Envelope>
Note that the previous request is only valid if you belong to the FedEx CSP program, if you don't, the code below is the right one for your:
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body>
<TrackRequest xmlns="http://fedex.com/ws/track/v6">
<WebAuthenticationDetail>
<UserCredential>
<Key>DEVELOPER_KEY</Key>
<Password>DEVELOPER_PASSWORD</Password>
</UserCredential>
</WebAuthenticationDetail>
<ClientDetail>
<AccountNumber>ACCOUNT_NUMBER</AccountNumber>
<MeterNumber>METER_NUMBER</MeterNumber>
</ClientDetail>
<Version>
<ServiceId>trck</ServiceId>
<Major>6</Major>
<Intermediate>0</Intermediate>
<Minor>0</Minor>
</Version>
<PackageIdentifier>
<Value>TRACKING_NUMBER</Value>
<Type>TRACKING_NUMBER_OR_DOORTAG</Type>
</PackageIdentifier>
<IncludeDetailedScans>true</IncludeDetailedScans>
</TrackRequest>
</soap:Body>
</soap:Envelope>
Best!