There are a number of reasons you get gaps or spaces between your new sod but the main reasons are either because of lack of water when the sod is laid and attempting to root, walking on the sod before it is rooted and while it is wet, the sod was laid late in the year and did not grow together because again, it lacked the proper water, or the lawn was not fertilized within the recommended timeframe and didn't grow together like it would have had it been fertilized.
First, your lawn must be fertilized within 4 weeks of it being laid and after the first cut. The fertilizer will always help the grass grow together and will get rid of the spaces or gaps. Many of our calls because of gaps or spaces are from customers who have not fertilized their lawn properly or haven't fertilized at all. Even if your sod is laid late in the year, it should be fertilized (as long as there is no snow).
There is also the option of placing small strips of grass in the spaces and allowing them to root. This is best done in the early spring or start of fall, before the weather gets too hot. The small strips will die if it is done during the hot months.
In time, all grass grows together. It just needs the proper care.