Sounds like you are not charging your DSE601 cells for long enough!
Every 3 cycles I just let them charge for 12 hours and it keeps the cell capacity at maximum. I have noticed that if/when I just keep charging them ONLY until the LED changes color....they start to LOSE capacity. There are MANY products that come with chargers including instructions, stating that when the LED color changes, the charging is done. Most of the time, this is PURE HOGWASH. The LED changing color USUALLY means the current draw from the charging circuit has DROPPED below a certain level.......but MOST chargers are still charging at a much lower current rate, known as "trickle charging". Please try this and tell me what happens........for the next 2 charge cycles, leave the cells in the charger for 10-12 hours....then SEE FOR YOURSELF if cell capacity improves. Don't take anyone's word for it...try it for yourself. Don't discharge lithium cells anymore than you must. When you observe that vapor production is suffering....REMOVE the battery and put it in the charger....these are not like Nicads which NEED to be well discharged before recharging (cycled). On the contrary, they should not be discharged too much before recharging if you want to get a good lifespan from them. 2 batteries is definitely NOT enough if you really enjoy heavy vaping on a DSE601!!! You need at least 2 chargers and 4 cells, so you can LET THEM CHARGE for 10-12 hours. This is exactly why I have 4 OEM cells, 2 OEM chargers...PLUS the UltraFire charger with 4 cells. Try the longer extended charging times and let me know if your cells improve.
Got my new UltraFires and I'll give it a shot. Does that mean my OEM batteries are trash now cause I've been pulling them off the charger when it goes green? I've always got one on the charger overnight though.