The first invasion of Britain in 55BC was unsuccessful due to a wide amount of circumstances.
1.1 Bad Landing
Much as Emperor Caesar had crossed the channel by the correct route, he had landed in an incorrect region of the mainland. This meant that the Romans were now about to attacked by the native Celts due to the Celts being forewarned of the coming of the Roman fleet
1.1.1 Movement around the coast.
After this faulty landing on the beachfront, he then sailed a little around the Far South-East of Britain. Here, he was attacked by the Celts twice and fended of both, the latter of which suffered few losses on the Celts' side thanks to them being on chariot and horse and the Romans not having theirs even arrive yet!.
126.96.36.199 Wrecked fleet
After managing to defend the beachhead the Romans had now set up, they were then approached by the Mourini, people in the area of the Romans' beachhead who were asked to organise a peace between the Romans, who were originally happy with the terms of returning any Roman hostages. Then they caught wind of the damage that the fleet had suffered, the one coming in from the continent, over the course of the crossing due to storms, thus preventing their crossing. Also, the fleet they had their was also greatly damaged. Noting this, Caesar changed his mind on agreeing to the terms.
1.2 Lack of Provisions
Caesar had assumed that the invasion of Britain would not take too long. Of course, he had plenty of provisions, but a fair bit of them were on the fleet that had left due to the bad weather! So, due to this, the Romans had to forage near the heavy guarded Celtic settlements. This meant that a lot were killed by the Celts and few survived with any resources to speak of. Howvever, one occasion was noted down by Dio Cassius several centuries after the invasions, in which Caesar had come to one of the raiding parties' rescue!
1.3 Long amount of combat
Caesar had fought for several months after the beachhead landing. Eventually, after not much had happened in the way of progression in territory, winter was coming. As Caesar had now been away for a long amount of time, Gaul was now starting to act up and so couldn't get the troops he need from the continent. On top of all of this, provisions were at their lowest. Due to all of this, Caesar decided to call it a day and sort out Gaul. Caesar did get some hostages back, but not all. In the end, there wasn't as much successful things to say about the 55BC invasion as there unsuccessful things.
2 Second invasion's success
After 55BC, Caesar wasn't especially happy, but he was still glad that he tried. Rome as a whole thought this was the greatest laugh they had ever heard or seen! He made several overhaul changes to the battle plan and set out for Britain. What he achieved was mediocre to say the least, but was still a success nonetheless!
2.1 Better boats
A big problem from last time was the fleet's inability to withstand the crossing from Britain. To sort this out, Caesar had built a splice of the army's boats and the trading cargoboats, as this combination allowed for the ships to last a substantial amount of time in rough waters and to also manage to survive being high and dry on the beach.
2.2 More landings
As Caesar had more boats this time around, he could land in more areas, creating even more of a problem for the Celts and being able to consolidate a beachhead far more readily with his larger army.
2.3 Battle recovery tactics
Caesar and the commanding officers had become more apt at dealing with the intelligent Celts. For example, after having fended off the beachhead and making headway into Britain, the Romans were lead to the stockades the Celts were using to defend themselves. The Romans were on the back foot when the Celtic cavalry came rushing into the Roman ranks. The commanding officers told the Romans to split the huge block they were in and separate into two halves, allowing for the cavalry to pass through. From here, the Romans then threw projectiles into the middle and killed most of the cavalry, allowing them to win the battle.