Another NewBee question

[Suicide Junkie]

You may have given your cruisers orders to "Attack (Seek after) Planet X"

This does not get cleared once the target is reached, and after the failed assault, when you order the retreat, your ships orders are now:
1) "Attack (Seek after) Planet X"
2) "Retreat (move to Coordinates Y,Z)"
The ships thus attack again.
Be sure to clear orders before giving the retreat order.

As for the troop transports, forget weapons, and layer on the shields/armor.

Typically, the warships will attack until the planet is defenseless, and will then wait for the transports to move in.

Question: Do your troops get defeated immediately upon landing, or do they stalemate during combat, and be defeated at the end of the turn?
IE: was there two ground combats in your log the turn after or only one? (The first should be just before the battle log message, and if a stalemate, the second will be right at the end of the log).

If your troopers are not defeated on the first GC rounds, you could send more transports to attack simultaneously.
Otherwise, you need either a bigger transport, more shields on your troops, or both.