We've been on gavage. The breast used to be full and firm, sometimes …

We've been on gavage. The breast used to be full and firm, sometimes milk would flow out by itself when the baby was feeding, it flowed straight out with force. But for two days now, the breast is soft, like there's little milk in it. The baby strains really hard when he feeds. What could this be from? ? I feed him every 2.5-3 hours as usual.