from a shortboard to a longboard

yesterday was my first experience with a longboard in awhile. it was about chest high and a little choppy. the board was a single fin, at least 10 feet long and about 3 inches thick.
paddeling and catching waves was no problem. i could go straight towards shore, but turning was impossbile. i could drop in just fine, but as soon as i tryed to turn the board to go down the line i fell right off. i tried to turn the board the same way i did my shortboard (I'm quessing this is where i ran into problems). was it the fin setup, cause i heard single fins are only good in smaller surf? or do you have to do a drop knee turn. any advice/help would be appreciated.
paddeling and catching waves was no problem. i could go straight towards shore, but turning was impossbile. i could drop in just fine, but as soon as i tryed to turn the board to go down the line i fell right off. i tried to turn the board the same way i did my shortboard (I'm quessing this is where i ran into problems). was it the fin setup, cause i heard single fins are only good in smaller surf? or do you have to do a drop knee turn. any advice/help would be appreciated.