Late Roman Army during the 5th century

[Flavivs Aetivs]

The East was only sending ships, not troops.

Then why was the operation cancelled? That only would've been necessary if troops had to go back east, to fight Huns. Ships weren't need for that.
See Goldsworthy How Rome Fell. On pages 329-330 he makes it clear that Theodosius II sent "substantial number of troops" from his eastern army to try to help the west regain Africa.

And what was taking so long was restoring the field armies. Look how long it took after Adrianople - 2/3 of the Army of the East was destroyed (about 80,000-100,000 men

Which must've been largely due to a reluctance to serve by then. It didn't take long to rebuild the army after Trasemene. Or even Cannae--they again engaged Hannibal in 212 and 209 BCE in Italy. I don't think the western regular forces (comitatenses) were ever rebuilt after the desertions of 408. (By 420 they had "regraded' limatanei into pseudocomitatenses but they don't seem to have been effective.) Subsequent campaigns relied heavily on barbarian allies such as the Visigoths in Spain 416-418, the Huns in the 430s etc.

The Desertion of 408 was approximately 30000 gothic/Amaili/sarmatian Foederati (and probably their nuclear families judging by the size of that number) captured and put into service after the defeat of Radagasius in 406 by stilicho. They weren't the professional field troops (which Constantius III and Gaudentius Aetius had enough of to defeat the Goths and fight the various usurpers in Spain and Gaul, where Gaudentius supposedly died).

EDIT: Also If I can find the source I think there were like 1100 ships the East had sent to help Aetius, detailing that it was enough for 70,000 men and thier horses and supplies or something. If I can get my source on that, 70,000 men is twice the size of the Thracian and Praesentalis 1 field armies, which total would have numbered 30,000. Take into account that the vast majority of horses died when sailing (an example is when symmachus writes how he had 8 prize horses shipped from Spain and 6 (I think) died) and then you have to reduce that total number down to about 15-20000 men from the west left to fill the ships. That alone would equate for most of the Gallic Field army of the time (which was mostly Pseudocomitatenses with a few Core Palatina units most likely, plus private retainers of various roman possesores)

There was indeed a reluctance to serve- it was a reluctance to serve a givernment that would not pay them. That's why the army served the Gallic landlords - they were hosued on their estates and paid. Aetius alone is recorded to have fled to fortresses on numerous estates in 432/433 before going to pannonia.