Can you post a photo?

Beyond that, perhaps the watering is the problem.

You wrote: "The plant is watered twice a week (wed/sun) 2 litres each time. The planter is 16" across and 29" deep."

Several things to think about and/or check:
1. Does the water rush out almost as soon as you pour it on?
If so, the potting mix has dried too much and pulled away from the pot. Thus, the potting mix and roots are dry.
RX: soak in tub of water 20 minutes max.
2. Is 2 liters twice a week too much?
If so, the potting mix will always be moist to wet.
3. Is 2 liters of water too little?
If so, the roots are dry.

To determine whether or not to water:
1. Ignore the calendar.
2. Instead, use one of several tests to determine when to water.
a. tilt the pot. Is it light weight (dry) or heavy (moist to wet)?
b. Stick a finger into the surface of the potting mix.
If the surface is dry, probably too little water, and/or too infrequnet.
If the potting mix is dry to a depth of an inch or so, water.
If the surface is still moist or wet, wait.