You might get away with teasing a few roots from the bundle but natives as a rule do not like their roots disturbed. How was the root ball when you removed it from the pot? I must admit, I'm guilty of upending pots at the nursery before purchasing and putting them back on the bench if the roots are massed and balled.
I once had a 'Summer Red' Euc. which flowered, and also grew, erractically and so I dispensed with it: growth wise it didn't seem to know what to do. Last year I bought by mail order an orange flowering one, and while it too seems to loose it's way a bit, I have managed to prune it to a nice shape. Hasn't flowered yet. You might check your soil, maybe it could use a little iron, try iron chelates, available any where plants are sold. Also remember we've had plenty of rain this summer and if your soil is not well drained that could be the problem. If it is well drained try a little fertiliser-for-natives.
It's not absolutely necessary to cut the seed heads or dead flowers away, that's only done to promote more flowers and growth.