I know this problem and even found solution for it.
This is caused by "+attempt broadsides" flag in objecttypes.tbl ,and makes ship turn and engage enemy using broadside tactics. It can be useful for battleships like Colossus ,but fails with attack units such as Aeolus ,Deimos or your ship. The solution for this behaviour is: Create "carrier-obt.tbm" in your data folder ,put there this lines:
#Ship types
$Name: Carrier
$Counts for Alone: YES
$Praise Destruction: YES
$On Hotkey List: YES
$Target as Threat: YES
$Show Attack Direction: YES
$Max Debris Speed: 150
$FF Multiplier: 1.0
$EMP Multiplier: 0.9
$Beams Easily Hit: YES
$Fog:
+Start dist: 10.0
+Compl dist: 600.0
+Disappear factor: 2.0
$AI:
+Valid goals: ( "fly to ship"
"attack ship"
"attack wing"
"dock"
"waypoints"
"waypoints once"
"depart"
"undock"
"stay still"
"play dead"
"stay near ship" )
+Accept Player Orders: YES
+Player Orders: ( "attack ship"
"depart" )
+Auto attacks: YES
+Attempt Broadside: NO
+Actively Pursues: ( "transport"
"freighter"
"awacs"
"gas miner"
"cruiser"
"corvette"
"capital"
"supercap"
"drydock"
"knossos" )
+Guards attack this: YES
+Turrets attack this: YES
+Can Form Wing: YES
+Passive docks: ( "support" )
#End
It's modified retail "Cruiser" profile without enabled broadside.
Now ,assuming you flagged it as "Cruiser" ,remove "Cruiser" from ship flags and put "Carrier" in it's place.
It should attack with forward weapon now ,the thing that prevented it from using it's main cannon was most likely the fact the target got out of firing arc too soon (it happend to me with modified INFA Zargeus).