Repair platforms don't target constructors. You have to target them manually if you want to repair them. Just like trade -and refinery ships, they don't regenerate their health (Except for the TEC faction, after researching the Civillian Ship Safety Act).

I agree, this doesn't seem right!  (And I said so in an earlier post.)

I believe the regeneration rate on constructor drones should be increased.  It is so low, that drones effectively never heal!

Currently, the best way to heal them is to scuttle them!  (After the enemy has left the grav well).  Then they just reappear with full health.