Without knowing any of the details of the specific kit, here's the main things to keep in mind and check...
From what I've seen of drip irrigation kits, some of the connections will be Hose Threaded (MHT & FHT for Male & Female Hose Threaded connections), and some of them will be standard Pipe threaded (MSPT & FSPT for Male & Female Standard Pipe Threaded connections). As an example, I've seen where the back flow preventer will be FHT x MHT and the presure regulator was FHT x FSPT and the filter MSPT x MSPT. The only way these can be connected is to have a FHT end of the back flow in the hose spiket, the pressure regulator, and then the filter. Any other order doesn't match the FHT to MHT and FSPT to MSPT.
Note that hose threaded connections are assumed to be sealed with a washer, so each HT connection should have a rubber washer for sealing. If it doesn't, then that's your problem. On the other hand, SPT connections usually do NOT have any sort of rubber washer to seal (though I have seen some with things like shower heads). If you seem to have a leak at a SPT connection, try getting some plumbers tape (white teflon tape about 1/2" wide) and wrap it around the FSPT threads about 5 to 7 times and then connect the things together.
Of course the other thing to keep in mind is that even though all these items are DIG, they might have changed the design and therefore the old kit may not go together the same way as the new kit.
Additionally, when trying to determine Hose Threaded v. Standard Pipe Threaded, the two will go together for a couple of turns, so you really need to look at the threads closely to make sure you are connecting the right types together.