I have a similar issue, although I think that it may have to do with the population cap.

Actually I figured it out not long after posting this. There is a second boarding.lua file in scripts that needs to be changed in order for it to work. Insufficient pop cap is also another reason you might not retain captured ships.

*Independent forces planet exsists.

*Me with OP boarding shuttles and 100% success chance
"It's free real estate."

I changed the price of the boarding shuttle but its still 10050 in game

Did you change it under the New Republic Version? There should be a generic listing and a New Republic listing for boarding shuttle under FrigateUnits. Make sure you change the price for the right faction. There should be a line nearby that says 'Affiliation=Rebel'

I've just done that I'll give it a try

I tried doing what you said sparten but it didn't work

Hopefully you backed up the unit xml folder so you can start over.
I've never changed the success rate but I know how to add them to any faction and give them a ridiculous shield amount so ive never cared about success rate.

Well for some reason the way I did it the shuttles seem to stop working after a bit. After like 5 or 6 battles using boarding shuttles i'll surround an imperial star destroyer hit the button and nothing happens. But I usually work around that by saving and reloading or quiting completely and coming back later.

I backed up the frigate xml folder plus I changed the base price of the boarding shuttle first before posting my question but that didn't work either so I'll give it another go but thanks for the suggestion sparten also I'm guessing for the factions you've just have to change the affiliation

Its under UnitCorvettes not Unit Frigates. The first entry is 'Generic Boarding Shuttle' scroll down a bit then you should see an entry for 'Imperial Boarding Shuttle' and right after that should be the New Republic version. You should be able to copy paste these lines from the generic entry under the New Republic entry.

<Build_Time_Seconds>15</Build_Time_Seconds>

the New Republic entry should look like this

<SpaceUnit Name="Republic_Boarding_Shuttle">
<Variant_Of_Existing_Type>Generic_Boarding_Shuttle</Variant_Of_Existing_Type>
<Affiliation>Rebel</Affiliation>
<Tech_Level>0</Tech_Level>
<Lua_Script>GenericGameObject</Lua_Script>
<Shield_Points>5000000</Shield_Points>
<Tactical_Health>500000</Tactical_Health>
<Space_Model_Name>RV_TRANSPORT.ALO</Space_Model_Name>
<Scale_Factor>0.5</Scale_Factor>
<Build_Time_Seconds>1</Build_Time_Seconds>

you should be able to paste the build time line under SpaceUnit Name="Republic_Boarding_Shuttle"

Dont forget that you cannot capture ships that you have also called before capturing.
Ie, i send out a ISD and use it to help capture the enemy ISD. You will always fail.
You can call in the same ship AFTER it says boarding was successful though.

But yes you have to go to unitcorvettes folder for edits. You usually always leave the generic files alone.

Well I've given my shuttles a lot of shield and health so their basically invicible. What I'll do is have a dozen shuttles in a fleet and a Corellian Corvette. I'll set the Corvette to jump in first and then bring in my shuttles in an among the enemy fleet that way I can capture several at once. I also changed the retreat time to 3 seconds so my captured ships dont get shot up. But after loading the first time it works for several battles then I'll go in and hit the button on twelve shuttles and only one ship gets captured. I don't get the notification that the shuttles have used all their crews and nothing happens. As I understand it the shuttles get five chances PER BATTLE correct? Not five chances then the shuttle is useless.

for the price do I put it next to the Affiliation

I'm not great at this since I'm asking