Do I remember correctly that Lucretius specifically writes about having difficulty translating Greek words/ideas into Latin... Or am I thinking of Cicero?
Cicero also definitely complains about it, mostly when discussing all the technical terms of the Stoics. From De Finibus 3.5: 'The most learned people [the Greeks], working in a language which is generally considered to be richer than our own [Latin], are still allowed to use unfamiliar terms when dealing with recondite material. All the greater is the allowance that should be made for us [folks writing in Latin] who are so bold as to tackle these topics for the first time.' (trans. Woolf). Of course, he then claims Latin is technically superior--he just has to invent a lot of new words himself.