Check if streptomycin is still active. Can you write some details of your purpose ?
Even >1 ug/ml shouldn't give any colonies!
Like you want to select transformants or what?
How many bacteria you plating and which media you are using would be helpful for answering your query. Additionally how you are adding streptomycin, what solvent you are using for streptomycin.
Take some of those colonies which appear on agar and put them in STR containing broath and observe next day if those grow there also ?
So there are two probabilities at this moment: 1) your bacteria is not good enough. 2) Streptomycin stock is not good.