It is not a problem of voltage. It is a problem of current and power. Whatever its nominal voltage, your bulb has a power to be known in watts. If your $\ce{Zn/Cu}$ cells are not able to produce the required current, the bulb will never be able to emit light. And the current is proportional to the surface of the zinc plate.
So first, determine the power of your bulb in watts. Divide it by 2.5 V. You will obtain the nominal current in Amperes. Measure your current with one of your arrangements. You will probably measure less than the nominal current. Then try to increase the dimension of your zinc and copper plates. Or make up a second arrangement of cells, parallel to the first one.
Also try to put the two plates as near to one another as possible. This will decrease internal resistance, and increase the current.