9v battery voltage drop and regain its voltage
Most batteries under load will drop their voltage and the voltage will then go back up when the load is removed.  The voltage will drop especially quickly if the load is high or the battery is close to being dead. 

So you likely have one or both of the following:

1) Your load is way higher than 42 mA

2) You are using a battery that is already exhausted or perhaps you are using a carbon zinc battery or "heavy duty" battery?  They only have a capacity of about 50 mAh!  Even Duracell 9 volt only have approx 300 mAh, and some energizer are closer to 450 mAh.

As mentioned in another answer, using a battery holder with 6 AA batteries with last a lot longer as their capacity would be in the 2000+ mAh range.