without seeing any code, this is pure guessing, but have you tried a googlemap refresh right after removing the pins and before adding the new ones? Also, if it is the images that change, then you will need to give them a unique name (e.g. adding DateTime.Now to the filename) else your browser will cache them.
On a side note: are you aware of Googles 15x price increase? Since last month you can only make 28000 free requests/month (used to be 25000/day). You making a call every 15 sec (or two if you need to do a refresh to remove them first), you could reach your limit in 5 (2.5 in case of the remove refresh) days for one single user: you would potentially make 5760 (11520 in case of the remove refresh) calls per day per user. After using your free ones, it will cost you $7/1000 calls. This could get very expensive very fast!