No, it is not because the class is partial: the only thing the "partial" keyword does is letting you split a class (an interface, a struct) among two or more compilation units
]. This is especially useful when parts of your class are generated by a tool.
In other words, keyword "partial" is purely a compile-time construct: once a class is compiled, there are no detectable differences between a partial and a non-partial classes if they are identical in all respects except being partial.
The only reason why a compiler could not "see" a declared constructor is because that constructor is not public, and your code is not allowed access to it. Could you post the details of your error, and a code for the constructor's header?