I was fighting in Arlington: Asteroid F-1 and I noticed there is a very lengthy delay before another enemy ship spawns.
I might suggest you Read the September Update post I made a number of day ago Project Terran September Update
Changes were made to the Respawn rates of Asteroid Fields after listening to player feed back.
I have checked this morning and the standard amount of ships for a Law Level 11 has respawned in that field as suspected it just took some time.
Coops
At least this report confirms that the new respawn rates work as expected 