The Tartan Army signed off from four game international break with just one win on the board for new boss Sebastien Pocognoli and Robertson was in no mood for excuses at full-time at Hampden Park.
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"It just was not good enough. That's the whole team, the whole squad.
"We played OK in some parts, but we did not do the good things we did in winning over North Macedonia.
"We have to now go away and find the reasons for this.
"I don't know if it was down to tiredness, physical and mental. But, Slovenia had the same schedule as us and they looked much fresher than us."
Scotland have two games on the horizon in the November break at home to North Macedonia on the 13tth before ending the campaign away in Switzerland on the 16th.