Boro boss Tony Mowbray is determined to bring in at least one new striker, if not two, before the transfer deadline shuts on September 2nd.
Boro made an initial £1million offer for McCormack last week, before coming back with an improved deal in the last 48 hours.
However, Sky Sports understand Leeds have rejected their second offer as manager Brian McDermott remains insistent that he wants the Scotland international front-man to remain with the club.
Boro remain hopeful they can prise McCormack away from Elland Road but it remains to be seen if they will return with a third bid.
Meanwhile, on the back of Boro's interest in new front-men, Mowbray has conceded that one-time Swansea target Marvin Emnes could be available.
"I don't know, let's wait and see with Marvin," said Mowbray when asked about the Dutchman's future, after he was dropped for the weekend's win over Charlton.
"I haven't got the answer for you on that.
"Marvin has just got to get his head down. I didn't think he performed well enough in the last game for us to play him at Charlton.
"We brought Albert (Adomah) in, I wanted to play a different system and we got on with that.
"I don't know if it's possible if he could depart. We have only got two strikers at the moment, three if you count Curtis Main, who is a young boy.
"Wait and see what the next few weeks bring."