longboard slipping on bigger waves

I just picked up a 9'10 long board with a single fin.
I took it out yesterday on 4 to 5 surf, decent size, steeper faces.
I normally surf a 9 foot board that I can pump up and down the face easily.
This new board kept slipping on the face.
I noticed the tail of this new board has more rounded edges.
Is that the problem?
Is the single fin the problem?
If I got a longer fin would that help?
Thanks
I took it out yesterday on 4 to 5 surf, decent size, steeper faces.
I normally surf a 9 foot board that I can pump up and down the face easily.
This new board kept slipping on the face.
I noticed the tail of this new board has more rounded edges.
Is that the problem?
Is the single fin the problem?
If I got a longer fin would that help?
Thanks