i had an issue with the support towers. i noticed that on level 10 of the first map the support towers stopped repairing all together. it made it impossible to survive.
They dont tend to just stop unless its a bug I havent run into yet, but after the later rounds when they start attacking back, after it repairs a tower the support tower goes into cool down before it starts on another towers repairs. Trick is to use the perks to increase there range and upgrade the tower itself, with the higher range you can have multi supports healing diffrent towers in the same area, and they also cover each other incase a support tower takes damage the other can repair it at a decent range
 "Forgemaster 2")
 "Midnight Towers")
They dont tend to just stop unless its a bug I havent run into yet, but after the later rounds when they start attacking back, after it repairs a tower the support tower goes into cool down before it starts on another towers repairs. Trick is to use the perks to increase there range and upgrade the tower itself, with the higher range you can have multi supports healing diffrent towers in the same area, and they also cover each other incase a support tower takes damage the other can repair it at a decent range