Ive had this potho for many years and have had times where a section of a long vine has the leaves yellow and fall but not to this degree. there are 4 vines as long as 4 to 5' long. they look good. the center as seen in the picture is what's bothering me. I had to remove 10 leave a yesterday and I see at least 7 more that will need to fall also. any suggestions as to why? and what can I do to stop this.