Brown spots on leaves

I've got a problem with a mother plant that i have been maintaining for the past 3 months. It appears over the last month or so there has been more and more leaves with brown spots on them. I have looked through the FAQ and other threads but i can't seem to find the same problem.
It is growing in miracle grow potting soil, it has been fertilized regularly and has also been flushed a few times in the past. It has been growing under fluorescent grow lux lighting and has a fan to keep the temp between 70 and 80
Recently the spots have been spreading all over the plant new and old leaves.
Have a look at the photo i took and see what you think.
any suggestions would be greatly appreciated.

With the miracle grow it could be over fertilized. Or ya could lack ferts. I would flush the plant. Then i would feed with a all purpose fertilizer. If ya over done it with the ferts the problem is rinsed out. If ya need ferts then after flushin them they will get a all purpose nutes and it will be fed.

If it is miracle grow time release it could be a build up of salts in the soil, needing a flush (that your previous ones wouldn't have gotten I guess), otherwise you could be root bound or have a ph problem. I would start by checking ph, what ph is your water before watering, and the excess when flushing? If those check out then what is the ph of the soil? If that checks out then check the container to see if the roots are coming out of the bottom, if they are what color are they? White is healthy, too many of them though (and it would look like a lot I think) and it could be bad, especially if there is no more new growth. If it is not a ph or root bound problem, then you probably need to up the nutes.
good luck!

I have the same exact prob and have been lead to believe that it is a Nute lock problem and that 1t epsom salt per gallon H2O should fix it right up