Like Becky said it looks to be a case of overwatering. I don’t like the looks of that soil either. It looks like it is too “rich”. The plant you have will do well in poor soil. What I am guessing is the the surface may be getting dry but underneath in the root zone the soil is still saturated. I am also going to guess that you don’t have a drainage hole in the pot? If not, water will collect in the stones at the bottom and as soon as the roots get into the water they will start to rot.
If it was my plant I would bare toot it, look for any damaged roots (rot) and remove them and then plant in a sandier soil mix. I also would put a fine screen over the stones to discourage the roots from going in too soon. I think you may want to set it outside to get more sunlight if the temperature is consistently above 45 degrees F. Be sure to water with rain water and not municipal water too.