svn merge not performing merge

We're constantly running into an issue where the svn diff produces a diff with (for example) 5 changes, yet after the merge there are only 2 of the 5 changes in the affected file (yes, just a one-file commit). Usually when this happens when the diff changes are 50+ lines off.

For example, the diff says the change starts on line 100, but if the change is actually now on line 155 (because of other changes merged in), svn merge can't (or doesn't) find the affected lines and therefore skips over / ignores the change.

Questions:

1. What flag can I set in svn to say "if you can't find all the things changed, throw a warning / error so I can fix it before proceeding"

2. Is there a wa开发者_如何学编程y for me to tell svn merge to search more than 50 lines for a change?

3. If there's no way to do #2, can I specify an external diff handler, and if so, how can I do that and still get the advantage of creating and deleting files/directories that's not included in the svn diff output?

Thanks!