Late Roman Army during the 5th century

[starman2012]

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.