Two potential issues could cause your series slats not to show continuity or to fire: shunt placement and cue wiring.
When firing series slats, make sure you place the shunt at the end of the slat. If you have multiple series slats connected, the shunt only needs to be placed on the last series slat in the chain.
The second potential cause using multiple series slats is not having the same cues wired across all slats. For example, if you have two series slats connected to a module, and you have cue #1 wired on the first slat, but not the second slat, this will cause an issue. This is because it's a series circuit, to allow the current to flow from one slat to another you must also wire cue #1 on the second slat. If you do not want to place an igniter in cue #1 on the second slat, you can place a jumper wire to connect (short) the empty cue; placing a jumper wire is as simple as connecting the red and black terminals using a small piece of wire.
For more information on series wiring, please see Continuity Clarified.