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Without seeing the specific code, it is difficult to provide a specific answer. However, some common reasons for inability to deduce the template type in a C++ function template might include:

1. Missing template argument: The function template may have been defined with fewer template arguments than needed, which can prevent the compiler from properly deducing the types.

2. Ambiguous or conflicting type information: The function may be called with arguments that have multiple possible types, which can create ambiguity and prevent the compiler from deducing the correct template type.

3. Unresolvable dependencies: The function may have dependencies on other types that cannot be resolved at compile time, such as third-party libraries or headers that are not properly included.

4. Incorrect syntax or usage: The function may be defined or called incorrectly, which can cause the compiler to be unable to properly deduce the template type.