I will second everything that StevO said:
1. It does not sound like a fin issue.
2. Taking off with a longish board in small hollow waves can present pearling issues until you get a few important skills dialed in.
3. The first skill is improving your fore and aft movements on the board. To get the board to catch the wave, it needs to be slanting down the wave face, which means that your weight needs to be fairly far forward. Sometimes I am standing up near the nose ahead of the pad to pull into a stubborn wave. However, once you have caught the wave and the board starts to slide down the face, you need to quickly move way back on the board for two reasons; if you don't you will likely pearl, and you need to be back near the tail of a longish board to get a good turn down the line which is the real key to not pearling. As a minimum, once the board starts to drop in, your front foot should be just behind the handle, and you should be in a long surfing stance so that your rear foot is back near the fins. This needs to happen in that brief moment between the board catching the wave, and the board reaching the transition to the flat at the base of the wave. The smaller the wave, the less time you have to do this.
4. A common mistake when people are learning on small waves, is taking the drop straight down onto the flat at the base of the wave before initiating their turn down the line. If the wave is at all steep at this point, there is no way that a longboard will fit into the tight hollow at the base of a small wave without either pearling, or stepping so far back on the tail that it will kill all the speed. What you want to do most of the time on a small wave, is turn midway down the face, so that your board is tracking down the line while still up on the wave face, without ever hitting the flat at the face of the wave; this should mean no pearling problems. You can drop out onto the flat for your bottom turn, but only if the situation allows the board to fit into the curve; you caught the wave really early so that the drop is not steep, it is a really mushy wave without a sharp hollow at the base. a shorter board with tons of rocker and nose scoop, a larger wave so that the tail of the board is not being pushed up by the lip while the nose is already hitting the base of the wave. In small waves that lack power, you are almost always better off making your turn down the line before the board reaches the bottom of the wave in order to stay up high on the power portion of the wave.
5. Not everyone agrees on this, but I would recommend paddling for the wave in a staggered or "semi-surfing" stance. This will allow you to shift your weight fore and aft from foot to foot while you are paddling for the wave, and it makes shuffling fore and aft a little easier. If you are using a flat water side by side stance when you are paddling for a wave, it will increase the problem you described of being either too far forward or too far back, since you cannot shift your weight from foot to foot to make adjustments.
Hope this helps. As StevO said, once you are ripping down the wave and wanting to play with your boards turning characteristics, that will be a fun time to experiment with different fins and set ups.