I no longer use the toner transfer method to produce my PCB's(I use UV exposure), I remember using "Orange scented household cleaner" as an adhesive to help the toner to adhere to the PCB with consistant results.
Orange scented spray household cleaners contain Citronella Oil. This is a slow solvent for the Toner when heated.
Prepare to PCB as usual, spray a small amount of cleaner on a paper kitchen towel. Apply a thin film to the PCB and carefully rub with a a clean paper towel until you have removed the cleaner(no trace on the paper towel). Iron as usual.
The fine layer of cleaner left behind after wiping clean dissolves a fine top layer of the toner and "sticks" the toner to the PCB.
A point to note, when removing the cleaner check the paper for any black deposists. Deposists indicate that the board still needs cleaning properly.
Hope this helps, chester