Do you have a chronology for all these bits? As in what order you're expecting things to unfold. And what is "the middle bit" for you?
If you've set up a good beginning, then the middle should kind of unfold on its own, riding on the inertia and the set-up of the first bit.
Your question is too broad and you give us little to no info on plot and important elements. The obstacles (conflict) depends entirely on your story and the points particular to it. If you're vague we will only be able to give vague answers, which are no good for you.
With that said, and generalising, the middle is the meat. Despite what I said about riding on the inertia of the first bit, it still has a lot of wiggle room (unlike the last bit, which should be pretty much moving on firmly laid out tracks, excepting the odd twist here and there), and big new things can still be introduced (sparingly). By the start of the middle bit you should've introduced important characters, the inkling/foundations of the primary conflict (the long-term payoff), and several budding mini-conflicts, which are more personal and immediate, but that tie in thematically to the main conflict. If the intro is the fuse, then the middle is the dynamite. All those clues and whispers and what-ifs and questions you've ignited in the intro should explode in the middle--this is not the same as the climax, which comes later. An explosion is chaos, debris, confusion, hurt, high stakes, losing, etc. Here is where you mangle/destroy what you've built up in the intro, and rearrange it to keep the reader interested. Show the ugly (the world's ugly, the MC's ugly, the villain's ugly) which in turn motivates change. Everything is about change.
In my own case, I will have several conflict options laid out in front of me, and will simply pick the biggest obstacle, the most dangerous, the most annoying, the one with the highest stakes, the one that affects the most characters (directly or indirectly). Literature is about extremes. No one wants to read a mild story:The bomb doesn't only explode, it also opens a breach in the gas pipes below and suddenly the whole block has 1 minute to evacuate before it is levelled, and behold, there's a baby trapped in a burning room next to the MC. And then, the MC does the unthinkable: he decides to run away instead of saving the baby. Boom. External conflict, internal conflict, and you have an element to play with in the future that has changed the MC forever (guilt) and motivates him to change further, which in turn keeps the plot moving forward.
PS: Beware of introducing obstacles out of nowhere, a la diabolus ex machina. If you struggle to present a conflict in a logical context building on what you previously had written, then it is possible you'll need to reconsider the story so far, and plan ahead more.
That's the vague answer
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