The pushover curves (base shear- drift curves) depend on a lot of input parameters such as dimensions of the section and member, sizes of reinforcements, characteristic strength of concrete, yield strength of steel, Poison ratio, shear and elasticity modulus,.....Therefore I recommend you, first to check the inputs, if they are correctly implemented you may do a parametric study and by changing the parameters one-by-one you will observe the variation and you can judge that your application was right or wrong, your expectation was a real expectation or not.

I wish you the best of luck.

please check the cohesive parameter value and the concrete properties.