Why won't my simple OR gate work?
This is down to Boolean algebra rather than electronics. We have for your initial circuit with inputs \$A\$ and \$B\$ and output \$Q\$:

$$Q=\overline{\overline{A\land B}}=A\land B$$

Where \$\land\$ represents AND and \$\overline{\cdot}\$ represents NOT. So in this case you have created an AND gate. For the second circuit we have (where \$\lor\$ represents OR):

$$Q=\overline{\overline{A}\land\overline{B}}=A\lor B$$

By De-Morgan's laws. Therefore it is a functional OR gate.

Notice that the crucial difference between both of your expressions is the first one is negating the entirety of the NAND output whereas the second one negates each of the inputs before NANDing them.