Why Matlab cant find one of the first maximum
Because matlab wants it to be truly a local maxima, that is it needs to be preceded (or followed at the right edge) by lower values. If it's critical that matlab identifies the first (or last) element as a local maxima if the initial (final) slope is downward (resp. upward) then surround your array by -Inf:
A=[12;12;4;65;43;34;6;7;7;3;768;324;564;23];
ismax = islocalmax([-Inf; A; -Inf]);
ismax = ismax(2:end); %to remove the -Inf from the result
