Pressurize your keg and spray it down with star san solution or soapy water. Look for bubbles, especially around the lid o-ring, the pressure release valve and the the base of the posts. Lid o-ring leaks can usually be fixed by slathering it with keg lube. Pressure release valves can be tightened, but if the o-ring wears out on those you have to replace the whole assembly. If the base of the post is leaking and you've already replaced the o-ring, take it apart, lube the o-rings and reassemble. Tighten with a wrench, but take care not to over-tighten. Firm but not he-man strength will get it done. Leaky poppets wouldn't cause a loss of pressure while it's hooked up to couplers so that's not likely your issue, but once again lube the o-ring and if that doesn't work replace the assembly.
In regards to your current batch, I wouldn't worry about it. Once you get the keg holding pressure just carry on as usual to carb it up and serve.