Hi, jayanthi:
Yes. Your antenna is radiating strongest at theta about 50-60 degrees. Why is it radiating the strongest at that angle? It really depends upon how the current distribuning on your structure. It is hard to give any simple explanation. The current is distributing in such a way that the radiation at the angle is the strongest. If you change your antnena configuration, it may change it becaues it will change how the current flows.
For the pattern at theta = 0, it is due to the surface wave along the infinte substrate defined there. If you assume finite substrate, the dip will be gone while the overall shape is still the same. Anyway, for your particular antenna, the radiation is strongest at theta = 50-60 degrees. It is not bad at 45 degree because the beam is quite wide.
Best regards,