"Are the machines new?"
Translation:Jsou ty stroje nové?
Stroj is a masculine inanimate noun. The "matching" adjective for the nominative plural stroje is nové. Noví is used with masculine animate nouns in the nominative plural.
"Jsou ty mašiny nové?" was marked incorrect. It seems it should be equivalent to use of "stroje," no?
"Jsou ty mašiny nové?" is accepted. So... either that wasn't exactly what you wrote, or the intermittent grading bug is at work here. Unfortunately, unless you use the Report button to send your answer to the system, we can't tell you why it was rejected.