I am a big daycare supporter, and we have lots of great local businesses that I regularly recommend, but this is a request to owners to resist popping unconfident young dogs into daycare with the expectation it will "socialise" them - here's why...
Firstly your key socialise phase tailed off around 14-16 weeks and if your puppy wasn't confident enough then for free group play, neophobic adolescence really isn't the time to heap on the pressure. Secondly we don't know if the unconfident puppy may be caused by some learnt history, discomfort or genetics, and being in a space where they can't avoid play could make them learn that play / interactions are sore or overwhelming. Unhelpfully young dogs will often use play as a way to appease with unfamiliar dogs, so seeing a young dog use play behaviours doesn't always mean it's true play - yup, confusing!
The additional problem is that it can give owners a false perception of their dogs progress and confidence. As mentioned, the dogs may play and interact when in the daycare facility but is that because they really want to or because they are in an enclosed space and have no choice. I regularly see dogs for behaviour treatment who "love daycare" but are gradually becoming more over-excited or grumpy with unfamiliar dogs on walks. From a behaviour point of view, over-appeasement (pulling towards other dogs, excessive wiggling, low posture, excessive play requests) and grumpy behaviours (tense body, air snaps, growls, grumbles) come from a similar state of mind and suggest a dog feeling uncomfortable with the interaction. And unfortunately the over-appeasing youngster often grows into the grumpy adult if we don't intervene early and change their view of the interactions. And this isn't just a result of group play, nervy young dogs that are regularly walked in busy doggy parks, will practice rushing over to all dogs and may go the same way.
Daycare is a great resource when set-up correctly, with well trained staff, small matched groups, a schedule of regular downtime and positive play interrupters BUT, even in the best run facility, the enclosed nature means it's best suited for relaxed, forward dogs with a lot of natural resilience or good experience. If you do plan to use daycare for your adolescent dog, either get them used to it from a young puppy (if confident enough at the time) or look for a facility where dogs can be picked up for group walks and then dropped home. Being around dogs, but in open spaces, is much easier for a new dog as the scents and sounds distract from the pressure to interact. It also limits the dogs activity levels at a time they may already be dealing with some growing pains or similar.
I say all this not to be a killjoy, I love watching dogs play as much as anyone, but with the aim to reduce the chance of seeing your dog for more serious behaviour work when they are older. So what should we do with mildly nervous young adolescents? Firstly it is good for them to meet friendly unfamiliar dogs, but manage their interactions, and keep them short and sweet. Secondly, and with dogs that they may like to learn to play with, let them get to know the dog first gradually through an onlead walk. Once the dogs consistently show more forward relaxed behaviours with their new budding friendship, then allow them to have play time but be ready to do regular recalls to interrupt the play. This will teach them to both like interactions and to trust it is on their terms.
Lastly, don't worry if your puppy isn't a social flutterby - all dogs are individuals, and in the same way we wouldn't throw a nervous toddler in at the deep end to learn to swim (i'm told that is not the done thing nowadays), we shouldn't do this with our dogs. Time and gradual, positive exposure is more effective than drowning.