Joanna,

I had this issue when i first tried soft agar assays. The soft agar is prone to detach with movement over time, especially when tilting the plate to aspirate old media and replace it with fresh stuff. I know you mentioned that you handle the cells carefully but it's possible that your media changes 2X a week are causing the detachment. Try once a week instead and don't touch your plate at all in between. Put it in the back of the incubator and forget about it until you are ready to change the media.

Also, sometimes the agar quality is just bad. Make some fresh stuff and see if it makes a difference. I usually make a stock of 3% and store 1mL aliquots in the fridge. From this I typically make a 0.6% bottom layer and a 0.3% top layer. These layer concentrations are easier to work with and should give you the same results. Just make sure the media you are using to make the layers is sufficiently preheated before you add the agar. 

Good luck and let us know if you figure it out.