Session variable lost after Response.Resdirect

attempting pass value page session variable being lost after response.redirect.  i've looking on google solutions said use:

response.redirect("yourpage.aspx",false);

however wehn try read variable page redirecting exception:

"object reference not set instance of object"

what things can check prevent session variable being lost?

is server.transfer alternative or query strings

is possible redirect in javascript , not lose session variable?

many thanks

response.redirect("checkout.aspx?factor=" + x.tostring());

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