Lava Lamp

Does anyone here have a lava lamp? My daughter has one in her room that she keeps on while she's sleeping. The past 2 weeks the lava is sticking to the sides of the bottle and the water is slightly cloudy. I had lava lamps growing up and this never happened to the ones I had. What could be causing hers to do that? She's had it maybe 2 years. The lava lamp place said it's safe to keep it running for however long we would like so that can't be the reason. Old maybe?