Unless your unit is defective, by default most DSLRs will not release the shutter if:

1. Focus has not been acquired.
2. The flash is charging
3. The buffer is full

1 and 2 can be over-ridden using the custom settings. If you go to MF or AF-C (which defaults to Release-Priority) you should not experience this due to #1. If the flash is down (and no flash is attached to the hot-shie), you your next experience this due to #2.

3 happens when the camera is waiting to write to the memory card. If you get a faster memory card, the limit can be pushed further but only up to a point. If will happen if you keep shooting consecutive shots quickly.

Unless your unit is defective, by default most DSLRs will not release the shutter if:

1. Focus has not been acquired.
2. The flash is charging
3. The buffer is full

1 and 2 can be over-ridden using the custom settings. If you go to MF or AF-C (which defaults to Release-Priority) you should not experience this due to #1. If the flash is down (and no flash is attached to the hot-shie), you your next experience this due to #2.

3 happens when the camera is waiting to write to the memory card. If you get a faster memory card, the limit can be pushed further but only up to a point. It happens if you keep shooting consecutive shots quickly.