I have Mandevilla plants in containers outdoors in Northern California. They are 5 years old and each spring and summer, leaves in the middle of the vine turn yellow and drop off. I've repotted them twice in new soil and larger containers over the years in case there were soil issues. These containers are 13" H x 25" W x 11" D. This didn't seem to fix the issue. They are insect free and get drip irrigation watering daily at 9am on a timer. The tips of the vines seem to be growing and there are plenty of flowers, but when so many leaves turn yellow and fall off it looks a little sad.
Is this normal growth pattern for this plant? If not, are there other things that I should try?