Your questions on Pitta, answered by Dr JV Hebbar  
Please explain the concept of high Pitta but low digestion strength (agni).  
In rare cases high Pitta and low agni exist together. 
Usually, in most of the cases, a gradual increase of Pitta dosha leads to an increase in digestive fire. The person starts feeling hungry all the time and there will be heartburn. 

But if Pitta is pathologically aggravated, then it gets over-burnt and leaves its original hotness and spicy nature. This leads to bitter or sour belching and decrease in the digestion strength. Here, the Pitta has turned pathological and has left its natural digestion promotion activity.  Pitta Dosha has liquid quality. Hence, it is often compared to a liquid fire. In some cases, the liquid quality of Pitta Dosha (water element) increases disproportionate to the fire quality. For example, when a person consumes excess lukewarm water. It increases the liquid quality of Pitta Dosha in excess, thereby causing a slight decrease in digestive fire. 
Read: Benefits Of Drinking Hot Water, Contra Indications
Ama is a condition in which erratic food habits lead to semi formed digestive juices mix up with undigested food to form a complex. When ama mixes up with Pitta dosha, then also, Pitta will be increased but the digestion strength will be reduced. 
Read: Pitta Associated With Ama - Symptoms And Treatment

There are five types of Pitta. What causes an individual increase of Pitta Dosha? 

There are five types of Pitta Dosha. 
Pachaka Pitta - aids in digestion 
Bhrajaka Pitta - responsible for skin color, complexion, absorption of ointments via skin 
Alochaka Pitta - eye and intellectual activity 
Sadhaka Pitta - determination, intellect, heart activities
Ranjaka Pitta - Blood production 

Among these, Pachaka Pitta is central and controls most of the activities of the rest of the Pittas. For example, if someone eats spicy salty food continuously, his Pachaka Pitta increases. This in turn causes an increase in anger (increase of Sadhaka, Alochaka Pitta), burning sensation on the skin, eyes (Alochaka, Bhrajaka Pitta increase) etc. 
Read: Pachaka Pitta Location, Functions, Imbalance, Treatment
However in some cases, individual types of Pitta can be locally aggravated. For example, when someone is exposed to scorching sun, his Bhrajaka Pitta may increase in the skin leading to local burning sensation on the sun exposed parts. 
Read: Bhrajaka Pitta – Location, Functions, Imbalance, Disorders

When someone sees very bright objects / smartphone continuously, his Alochaka Pitta may locally increase in the eyes causing vision problems. 
Read: Alochaka Pitta – Location, Functions, Imbalance, Disorders