Burning that begins at the tips is often caused by salt burn... if you are in the southwest and get your water supply from the salt river like we in az do then u know all about it.

Try flushing out your soil very well for like an hour. Just leave the hose there on a slow drip, make sure the water isnt puddling but being absorbed into the soil.

If you are watering the same amount every time and it isnt running well out of the bottom then the water is reaching the same poing in the pot every time accumulating a salt layer.

Just a theory :O might be somethin else.