Why can't I get transformer output as written on the label?
Your capacitor is far from adequate to maintain the peak voltage, and you're probably exceeding the rating for the capacitor ripple current. To get 1Vp-p ripple at 10A out you need about 100,000uF, assuming 50Hz mains. 

Also from a transformer rated at 12V 10A (RMS) you can expect to get a bit over 6A DC safely from the filtered DC output. That scales.