I'm able to import the PrivateKey successfully, but am not able to generate Signatures deterministically.
Great! That indicates it is working. If the signature were deterministic that would be cause for concern.
ECC (unlike RSA) does not produce the same output every time, nor does it produce the same length every time! ECC signatures can vary by as much as 5-bytes (sometime more). IE you might sign the same data 100 times and see signatures that range from 70 - 75 bytes in length. This is expected and common with ECC. I see you are already doing a sanity check using the verify to double-check the signature, that is good! Below are a series of signature generation/verification calls on the same exact input data I ran locally. (You can also use a third party verify operation if you want to triple check it.)
Hexdump: Signature of length 71 Should output 4 lines 3045022073e648793a98424f345c06f0a6 63f406b82a3321702e692c0d6926fefa 681183022100a2fb9275f9b61f253564 26c745e6be56e43f134953931b54edbd cefd802bf17c Signature verification was successful! Hexdump: Signature of length 70 Should output 4 lines 30440220106f09553c87c6a6622403503d 28b6469280d735207d29f614ec0da823 33608702204f0549bb242fb69e9939f5 f269f0720f624288ce9ae4d5dd27a304 fc84c63f8a Signature verification was successful! Hexdump: Signature of length 72 Should output 4 lines 304602210092aa287e79f309f686e98108 d11a0851b91ea9556e55954c7cfd2c49 97f6ca1c022100b3876fec6dcddc6546 a57d12fc0bd8e17930fd2af9f1049c77 fead7eba57e9c6 Signature verification was successful! Hexdump: Signature of length 70 Should output 4 lines 304402205268f74dec640c683a92bde39c a3125fa3b52cf54ab8f4439676a7bf9d 87a11a022065ab167156d580a328068f 1acd4de3fc35cf61e5b8df41c618466e a9637cea3c Signature verification was successful! Hexdump: Signature of length 71 Should output 4 lines 304502207819fa3c308d7337b13c232fc9 a0b4d6ae7d1432fcdbd57b116d1a9aef 6da855022100ad65c70fd7fec03b70a7 5708d1512f08c98e3b7ecf84b4974ab3 7d0c3053f0ad Signature verification was successful!