I am not sure which particular autocomplete situation you are having problems with.
If the names are in the symbols pane of an _open file_ of the same language they should appear for autocompletion if you type the first few characters (how many is settable in preferences, default 4).
Is that the problem you are describing, or is it that many names do not show up in symbols. The ctags system used to get the symbols can only handle named scopes, not lexical scopes, so it does not parse local definitions, so those names will not show in symbols.