I had a feeling you just might be using a weak plug battery...
A typical glow plug will pull 2-3 amps...maybe a little more? (depends on the plug, and if it's flooded or not, etc.)

You really should get a good Nicad rechargable glow battery, because those dry batteries just don't have the amperage delivery capability, and will croak very quickly...(IMO they're not worth bothering with )