Could be it's too thick for the retainer, or the retainer is filled with cr*p, or if it's cold there it's frozen, or there a fastener in it somewhere. Is it definitely acrylic?
If it is you'll have to use care, if it's lexan you can be a little more aggressive. Since you have two sides open have someone on each side and try working it out.
If the retainers dirty try flushing it out.