Deffo NOT spidermites...

you should start in a small pot then when its well rooted go to next size up,
eventually getting to ur 16ltr if that's whast u want to use.
Plant dosent look overfed...
maybe try letting soil dry out a bit before feeding next.
all I can think is starting in your 16ltr pot the roots wudnt have filled it much you may be drowning them a bit.
do you use a wet/dry cycle on the soil?