In a non-uniform electric field, the +q and –q charges of a dipole experiences different forces (not equal and opposite) at slightly different positions in the field and hence a net force F acts on the dipole. Also , a net torque acts on the dipole which depends on the location of the dipole which is given by 

Where  r is the position vector of the centre of the dipole.

When the dipole is parallel or antiparallel to E

In a non-uniform field, if    , the met torque on the dipole is zero because the forces on charges become linear. However there is net force on the dipole as shown in fig.

 

When dipole moment is parallel to E, a net force acts on the dipole in the direction of increasing  E. When   p is anti-parallel to   E , a net force acts on the dipole in the direction of decreasing E.