Why can not I use all CPU cores?

miner : cpuminer-opt-3.4.8-windows mining on vps

if -t 8

CPU: Intel(R) Xeon(R) CPU E5620 @ 2.40GHz
CPU features: SSE2
SW built on Sep 22 2016 with GCC 4.8.3
SW features: SSE2
Algo features: SSE2
Start mining with SSE2

[2016-11-16 06:46:54] Starting Stratum on stratum+tcp://eu.poolinat0r.com:4443
[2016-11-16 06:46:54] Scrypt buffer allocation failed
[2016-11-16 06:46:54] Scrypt buffer allocation failed
[2016-11-16 06:46:54] Scrypt buffer allocation failed
[2016-11-16 06:46:54] 8 miner threads started, using 'scrypt' algorithm.
[2016-11-16 06:46:54] FAIL: thread 60750360 failed to initialize
[2016-11-16 06:46:54] FAIL: thread 56556056 failed to initialize
C:\Users\Administrator\Desktop\cpuminer-opt-3.4.7-windows>pause
Press any key to continue . . .

if -t 4

CPU: Intel(R) Xeon(R) CPU E5620 @ 2.40GHz
CPU features: SSE2
SW built on Sep 22 2016 with GCC 4.8.3
SW features: SSE2
Algo features: SSE2
Start mining with SSE2

[2016-11-16 06:48:59] 4 miner threads started, using 'scrypt' algorithm.
[2016-11-16 06:48:59] Starting Stratum on stratum+tcp://eu.poolinat0r.com:4443
[2016-11-16 06:49:08] CPU #3: 12 H, 2.49 H/s
[2016-11-16 06:49:08] CPU #1: 12 H, 2.44 H/s
[2016-11-16 06:49:08] CPU #0: 12 H, 2.42 H/s
[2016-11-16 06:49:13] CPU #2: 12 H, 2.68 H/s
[2016-11-16 06:49:13] Accepted 1/1 (100%), 48 H, 10.03 H/s

cpu used 50%

why? please help me !