I had the same problem with my Fiskars rotary cutter... and I just dealt with it for a while. Then I changed the blade, and I discovered the problem fixed when I, for experiementing's sake, put it back together the opposite way. It worked. I apparently had it in the wrong way for a few weeks!

I've been using Olfas and Fiskars for years.
I bought a Martelli several years ago and could not the hang of it. A friend recently showed me - and boy it's a great rotary cutter - easy on the wrist and the blade is the best I've ever used. I just bought more thru Amazon.
I also recently bought a Gingher, nice hand, cuts well, but not as good as the Martelli.

Plumber's tape generally does the trick.

I had the exact opposite problem. I couldn't get the screw to come loose. I called the company, they sent me a new one. It took a week or so, but I was thrilled. I have an older one for back up so didn't go without while waiting for a new one to come in the mail. Love those companies with great customer relations.

Rotary cutters
 

What is the best rotary cutter to get? I'm lefthanded .
It seems that I have to go over my fabric several times to cut it completely plus my blades get dule very fast compared to other people that I now.

There is a product called Lock Tite. It comes in a tube in the adhesive section of your local hardware/home improvement store. There are two types. One is temporary and the other is permanent. One is called Red and the other Blue. I don't remember which is which so read the package. It works great. I used it for a keychain I have that has a screw on knob to hold the keys and charms on that kept coming loose. That reminds me,,,need to buy a new one. I have to add a new granddaughter charm to my keychain. The last tube got lost in the move. With the temporary one once the nut is put on it will stay where you put it and you will need a pair of pliers to remove it. It will not tighten or loosen so make sure you have the nut tightened to the right pressure by testing the blade before the glue dries. You will have to reapply each time you change the blade. I haven't used the permanent one so I would assume that if used the nut will not be easy to take off for blade change.

There is a little thin bent metal washer between the screw and the nut. I lost mine and had the same problem. I would call Fiscars and see if they will give you a new one. I didn't call because I had another cutter, but I plan to do so.

I've had Olfa and I've had Fskars. Fiskars is my choice for easy on arthritic hands. Cuts just fine.

But all of mine have to be tightened every so often. I just do it.

Did you perhaps loose that little plastic washer that goes on before the screw when you last changed the blade. I have to be so careful to get everything back in the right order, or the screw WON'T stay tight.

I must admit that I don't like the Fiskers cutters, Olfa's are OK but I love the Westcott ones, it helps that I can get them so cheap on EBay. http://www.ebay.co.uk/itm/Lot-Of-6-O...#ht_791wt_1093

Their blades are terrific, they last longer than the Olfa Titanium ones and I do a lot of cutting! They are so comfy to use, for left or right handed users.