Where the water enters the toilet ballcock valve - is it a "one-way" valve. Sometimes there is a devise incorporated to prevent a back-flow so that the water source cannot be polluted.
If so you will need a higher pressure to open this valve so will need a higher head of water.
In the UK we are suposed to have these devices fitted in showers etc. so that if the shower head falls in the bath, for example, you don't get dity water syphoned back into the mains.