They don't always  trade the same contract at the same time ... different products ... spend some time reading on this type of products  if you intend on trading them ... this will save you TONS of money in the long run  ... ;) 

https://www.uscfinvestments.com/filings/commodities/ung

Roll Dates are projected and subject to change without notice. Roll Dates are the expected dates on which the composition of the Benchmark Futures Contract is changed or ''rolled'' by selling the near month contract and buying the next month contract. The change occurs over four days.

UNG Roll start April 12th finished april 15th

https://www.proshares.com/our-etfs/leveraged-and-inverse/boil

The index is valued using the settlement prices for the underlying futures contracts. The index rolls its contracts over the course of 5 consecutive business days, starting on the 6th business day of the month. Each day, approximately 20% of each futures position that is included in the month's roll is rolled