Beginner issue or conditions just weren't right?

I tried out a 6'3'' long 18 inch wide 2 5/8 shortboard I borrowed from a friend a few weeks back. I really still just a beginner and just recently began to have some success catching some waves on a 7'4'' funboard. Regarding the shortboard, I found that everytime I went to make the poppup, the board would sink and wave would pass under me. It seemed that the waves weren't very steep this day, only 2 &1/2 to 3 feet high. It seemed that most surfers had longboards, minimals and some fish...I really didn't notice if there were other shortboarders in the water or not to be honest. So what I was wondering was is what I've described more of a problem with inexperience and my abilities or regardless of the experience level of the person...this particular shortboard just wouldn't work in these conditions?