When a lifter converts from conventional to sumo some break-in time is needed. With conventional deadlifts you use a lot more lower back and a lot less hips and glutes. With sumo
deadlifts you use less lower back but your glutes, hips, and hamstrings have to be super strong for you to be successful There are some exercises to help strengthen those hip flexors
The first exercise is the box squat. Put the box three inches below parallel. This is a very deep squat as most lifters train just to break parallel. Your hips will be recruited into the movement and this will really tax them. Start with a low weight and slowly increase it week by week. Don't try and be a hero by using heavy weights from the first week out or I guarantee you will injure your hip. Ease into the weights. Sit on the box for a two-second count, release your hip flexors, then explode off the box. This is one of the best exercises to strengthen your hips and glutes.
The next exercise you can do is the Zercher squat off pins. For this exercise the bar is held in the crook of your arms. Use a towel to help with the pain of the bar and to make it more stable. Most lifters do this off the squat hooks, squat for the specified reps and then return the bar. This is great for trying to increase your squat. But for the sumo deadlift you need a different approach. Set the pins in the power rack so they are low enough that when you are underneath, holding the bar, you are below parallel. Once you are in position you can just stand up with the weight-the bar in the crook of your arms. This better simulates sumo dead lifting form because it calls upon all the muscles you need to sumo deadlift, but it is much harder to do. This is because your leverages are reduced and you cannot effectively call upon helper and stabilizer muscles like your shoulders, biceps, upper back and traps. All the stress is placed heavily on the glutes, hamstrings, hips and lower back.
Try incorporating these two exercises into your program and within as little as four weeks you will see your deadlift strength increase. Do them once a week on the same weekday. During this period, stop all dead-lifting movements. After a month give your sumo pull a try. I know you will be happy with the results gained by your newfound hip and glute strength.