If loosening the nut doesn't help, then I think you have an out of
tolerance
collette and should return the router for a new one.
I trust more on the router than on the bits. From reading here Makita is a
good router maker. The stright bit is from Irwin. Made in China but it is
sold under the USA mark "American Tool". And that Irwin bit fits perfectly.
The bits that doesn't fits is sold without any commercial brand...