Depth of water level in a borewell in Bangalore found to be different the day after drilling- What could be wrong? Please advice

We have drilled a bore well in Ramamurty Nagar, Bangalore and have put casing at 160ft. We located water source at 400ft, went upto to 600ft, water source was around 1.5" to 1.75" as per the borewell operator and we stopped at 600ft. Next day we opened the cap & checked the depth, it stopped at 250ft, but water source was at 200ft. Please help and tell me as to what could be wrong. We are planning to install motor upto 500ft. Please advice.