Are they plain gauge spokes? These are more likely to loosen than butted ones, because they aren't as elastic and are more likely to lose all tension when you hit a big bump, especially if the wheel wasn't built very tightly to start with. The wheel can be trued, but I'd also add a lot more tension.
A couple of years ago, I bought some pre-built wheels because it was cheaper than getting the parts separately (this is sometimes the case). Deore hubs, DT Competition spokes and ceramic rims, all for about £100. They weren't bad but were a bit slack for my liking. I added another half turn of tension all the way round and then re-trued them.