In May, most public miners didn't produce significantly more bitcoin than in January. These companies have struggled to keep up with the increasing difficulty and have not been able to grow their hashrate as fast as expected.
Some of these companies are even producing less bitcoin now than at the start of the year. Marathon is the prime example, mining 462 BTC in January and only 268 in May, corresponding to a 42% decrease. Their May and April production numbers are especially disappointing considering that they had among the most extensive hashrate expectation plans.