26-year-old Spencer picked up a knee injury on Boxing Day as the Robins lost 4-1 to Swansea City.
Steve Holland told the official website The transfer was a no-go because his medical report showed that he was struggling with an injury.
"It was hugely disappointing for everyone involved because we had gone so far in the negotiations and he was just the player we were looking for. He ticked all the boxes we were looking for in a striker.
"It is disappointing that it is a no-go for now, but the situation could change in the future. The two clubs have a great respect for each other, so we are not totally ruling out the possibility of a transfer occurring in the future."