Most modern flash systems that employ a TTL metering method have some quirks. One of the major inconveniences that I have come across quite a bit is that when you are in a relatively dim environment the automatic TTL system usually gets a little overzealous attempting to balance the ambient light with the flash exposure.
This results in ridiculously long shutter speeds, even when shooting a subject that is easily exposed by the flash. This can be a bit of a hassle and lead to ghosted exposures, especially when shooting moving subjects or portraits.
The solution is your camera’s custom function menu. In the custom functions section of your menu there will normally be a setting that allows you to specify an exact shutter speed to “lock” your camera at when shooting TTL in the program or aperture priority modes.