How much force does a solenoid really have?
Force from a solenoid = \$(N\cdot I)^2 \dfrac{μ_0\cdot A}{(2 g^2)}\$

Where: 


μ0 = 4π×10-7
F is the force in Newtons
N is the number of turns of the coil
I is the current in Amps in the coil
A is the area in length units squared (cross sectional area of the coil)
g is the length of the gap between the coil and the iron.


If you want to check your solenoids pull force and you know the number of turns and have a good estimate of the "gap" and the cross sectional area of the winding then use the formula.

However, it's notable that force is proportional to current-squared and if the 12V you used dropped down to (say) 6V under the load of the solenoid, the pull force would quarter because the current would have halved. It's also worth noting that the force is inversely proportional to the gap-squared - double the gap and the force quarters.