It is possible it isn't locked in correctly. I have an M8540 with SSQA FEL/bucket. If there is any debris between ANY of the mating parts of the FEL and/or bucket, those pins will not go all the way down into the slots. Because the pins are spring-loaded on the levers, you may or may not notice. The levers should go down real easy without any noticeable resistance.
Once the pins are all the way down, there shouldn't be any moving and/or flexing.
When doing any work with your bucket (dirt, etc.) it is fairly easy to get junk in there. Once you remove the bucket, the dirt can get into places that it shouldn't be. I usually rub my hand up underneath the bucket lip (where the FEL hooks underneath) and where the pins go. After a little wear, you will see where the contact points are and where there shouldn't be any dirt. If there is dirt in there, it is taking up clearance.
In addition, once locked in I curl the bucket (or whichever attachment) back and forth real fast (mostly to the all-the-way-curled position) to shake it into place... in case there is anything bound up a little. This shaking usually moves things into the sweet spot where it wants to be.
The pins should be down to where the bucket slot is almost to the top of the taper. When bound by debris, only the tip of the pin will get into the slot... which is no good.