Answer
This is very often a whitespace mismatch between the $SEARCH block's text and the actual original source — extra or missing spaces/indentation that aren't visually obvious can still cause an exact-text-match $SEARCH to fail to locate its target. Copy the original code block verbatim (including its exact whitespace/indentation) directly from the Ext-layer source into the $SEARCH section, rather than retyping or reformatting it, when combining multiple $SEARCH/$REPLACE/$APPEND blocks within the same overtaken method.