Jmeter Results analysis

Question

Kindly i need a help in explaining some performance test results:

1. Load test total 800 user : we used 8 slaves each one run 100 users with ramp up time 100 sec, we expected 5195 samples (by multiplying number of total users by number of actions (requests)), but we got in the results 4252 sample and when we checked the csv file we found that only 649 user success to start executing the first action (Hit Url) while we were expecting 800 user to start executing the first action (Hit Url).

2. Stress test total 2500 user : we used 10 slaves each one run 250 users with ramp up time 250 sec, we expected 15095 samples (by multiplying number of total users by number of actions (requests)), but we got in the results 3185 sample (which is less than the samples we got in the load test) and when we checked the csv file we found that only 515 user success to start executing the first action (Hit Url) while we were expecting 2500 user to start executing the first action (Hit Url).

would you please help me in explaining this ? we not all users can start execute the first request ? is it an indicator for a performance issue?