Hi

Try adding an extra quantity of pure agar agar to the XLD plate, It may end the problem, and Repeat culturing again of your pure culture.

Be sure not to use anything that is contaminated, Try not to contaminate tools you are used and make sure of that

You said cream color I think you mean yellow , but  Escherichia coli on XLD appear Yellow colonies,{look the attached : source: (http://microbeonline.com)}

Thanks

Thanks for your suggestion. I am sure that's not E. coli contamination. And I will try extra agar on XLD. 

make sure the plates are dried before inoculating them - I assume you are using spreadplates?

For either spreadplates or pour plates make sure that the inoculum is evenly distributed on the plate - and if a spreadplate that the liquid soaks into the plate

Thanks for the suggestion Ian Jenson. I am using spreadplate and drying the plates thoroughly actually helped to get nice colonies.