The visitors thought they should have been given a penalty after Patrick Bamford took a tumble in the box in the 76th minute, but the Chelsea loanee was booked for diving.
Robinson felt it was a spot-kick but conceded the Blades were unlucky not to be awarded a penalty in the first half for a handball.
"I don't think anyone could've come off and been disheartened with that," Robinson said. "The frustration lies within that I thought we should've had a penalty in the second half.
"But, if we get that, then it's a little bit unfair on Sheffield United. But you do ask that key decisions are right.
"I thought they should've had a penalty first half and we should definitely have had a penalty in the second half."
Robinson was also left to rue his side's mid-table position after taking eight points off the top two in League One this season.
He added: "I'm pleased today; you've got to accept the draw because it was a fair result.
"We beat Sheffield United at home and we beat Doncaster at home and took a point away from home against the two of them and that's promotion form - that's the frustrating thing about it.
"The best two teams in the league, we've taken eight points in those four games. We should've done better in other games."