feval() and str2func() only look for functions in the "top level" namespace -- the functions with corresponding .m or .slx or .mdl or .mlapp or .mex* files. They are effectively executed inside the base workspace, not inside the workspace of the executing function. feval() and str2func() will not (reliably) find nested functions or functions that are private to the current file.
In practice what this means is that you should restrict yourself to function handles built with @ and not pass the names of functions to other routines as character vectors or strings, except in some cases where passing a specific function name is a documented option to the function. For example, cellfun(@length, X) and cellfun('length', X) are both valid and have slightly different meanings. See the "backwards compatibility" section of the cellfun documentation.