GNU Octave strives to be among the very best numerical tools available. While compatibility with Matlab is one of many strong features of Octave, we have not achieved 100% compatibility. If you are reading this page, it is most likely because you attempted to use a function that is part of core Matlab, but has yet to be implemented in Octave.
There can be several reasons for the given function not to exist in Octave:
The function may exist in Octave-Forge, but has not yet been adopted as a part of the core Octave distribution. You can try to look for your function in the alphabetical list of functions at the Octave-Forge web site to see if it is available in some package.
The function may be new to Matlab. It is possible that the Octave community simply does not know it exists. Should this be the case, feel free to open a feature request in the Octave bug tracker.
The Octave community is a loosely organized association of volunteers. Development might sometimes not be fast enough for your needs. If you have a feature request for Octave, the best way to ensure that your suggested improvement becomes a reality is either to implement it yourself or to hire a developer do it for you.