Why may LO pressure on both generator engines decrease?

The LO pressure may decrease along with increased temperature or due to the faulty LO pump. Check the LO temperature on both generators. If it is higher than should be, there is a problem with the LO cooling.

Since low LO pressure experienced on both AE at the same time looks like your engines lack of LO cooling. I this case, carry out the following checks:

1. Small Yanmar engines have LO cooled by the sea water. Check the SW flow through your Auxiliary engines. If SW flow is insufficient, high jacket cooling water and scavenging air temperatures should be experienced, as those mediums are cooled by the same seawater flow.
2. Seawater flow may be reduced due to a clogged filter of the supplying pump. Check the pump’s pressure, check it’s suction filter.
3. The FW cooler of the generator engine may get clogged. In this case, it’s outlet SW flow will be reduced. Sea water outgoing from the FW cooler flows into the LO cooler. Thus, if FW cooler sea water side got clogged, the LO and Scav Air coolers are not receiving enough sea water. Open FW cooler covers and inspect. Clean as required.