‘ And Yahweh was with Judah, and he drove out inhabitants of the hill country, for he could not drive out the inhabitants of the valley because they had chariots of iron.'
The hill country was permanently and solely possessed, but not the coastal plain. The idea is not that Yahweh could not, no such thought was in the writer's mind, but that Judah failed. God would only help them so far. This may have been because they were dilatory, or because of fear and lack of faith in Yahweh (compare Joshua 17:16 and note the promise for the future in Joshua 17:18; see also Judges 4:3). We should note that Joshua was not defeated by the chariots (Judges 11:9).
If Judges 1:18 is not read with a negative as LXX, it may be that this indicates that they succeeded at first in initially capturing three of the cities, taking them by surprise by coming from the south, but that their success was only temporary, and that then they were overcome by combined forces with their chariots, and after a time driven out of all. In this respect the non-mention of Ashdod is significant, and if the newly arrived and settled Philistines were expecting an attack from the East they may well have gathered their forces near Gath.