My springer's velocity dropped - what do I do?

Step 1: try a fresh tin of the right pellets and a clean barrel - leading or pellet quality is the most common cause.

Step 2: oil the piston seal with a drop of silicone chamber oil through the transfer port and shoot 10 to 20 shots; a dry seal loses velocity gradually. Step 3: if velocity is still down 100+ fps from new, the piston seal is likely worn out or the spring has taken a set, and the gun needs a reseal/respring (a basic tune by a qualified airgunsmith, typically $80 to $200). Do not attempt a teardown without proper spring compressor tools - springs store enormous energy.