The nature of Roman imperialism changed under the Principate. Under the Republic you had ambitious magistrates seeking battles in order to buttress their personal standing and their further election chances. This meant that Roman expansion wasn’t always pre-meditated or strategically logical.

Under Augustus this changed. Freewheeling generals leading armies became very dangerous. If one was to gain sufficient glory he already had an army and an opportunity to make a play for power. To mitigate this Augustus ensured that most of the armies were commanded by men he personally chose (rather than elected by the populace and assigned to commands by lot or the senate). However it still made sense to prevent one individual from amassing too much glory. So as a result you don’t see too much expansion under the Principate: the capture of Britain, Mesopotamia, and Dacia are the only two major ones.

As for Germania, it was intended to become a province until an ally led Varys’ army into a massive ambush. In the aftermath, Tiberius led numerous punitive expeditions to make the Germans aware that Varus’ destruction was an anomaly and that Roman arms were to be feared.

As for why wasn’t Germania retaken? Rome had recovered from numerous debacles before (Cannae anyone?) and now had an even larger pool of manpower to call upon. But the loss of three veteran legions was a massive hit which couldn’t be made up quickly. But the most likely reason was simply cost/benefit.

Dacia had a huge amount of mineral wealth. Mesopotamia was enormously wealthy as well. Britain was seized only because Claudius needed a major conquest to shore up his rule. When viewed against the costs of campaigning to capture a poor territory under a very strong emperor - it’s not worth the candle.

u/Agrippa911 is correct in that holding Germania would have been too costly for the small benefit it would have brought. If you want more detail on that point, please have a read of my answer here.