Yuo may have a dilution effect or some sieve effect. For the first try to concentrate by removing solvent under vacuum and respeat scanning near blue at different dilutions and for diffraction try to clean the solution by filtering or centrifugation checking if somethingh remains on filter or on tube end. You may also try to get a near UV spetrum with another uv sèectrophotometer is available in the range about 280 nm

Lower concentration in combination with lower extinction coefficient may give rise to lower absorbance and hence is not prominently seen in the visible region of the absorption spectrum.

By the way, what is your reference?

The colours that we see are not absorbed. The observed colour is complementary to the absorbed colour. So, even if the compound is coloured, it may have absorption in the UV region. For example, PACO, a beta-enaminone has violet colour but its absorption maxima is around 300 nm. 

This is the color observed. A standard protocol was followed to obtain this color, and according to different papers, the absorbance was taken at 510-520 nm. But, when we checked the lambda max for this color, there is no lambda max at visible region. It shows lambda max at UV region at around 310 nm.

What may be the problem here?

We see red even through the green bottles glass.

If you have a mixture of absorbing species or species with multiple absorbances then lambda max may not be in the visible. Black inks most often contain no black dyes or pigments, but are made up of mixtures of dyes and or pigments which combine to absorb all parts of the visible spectrum. If any coloured mixture contains dyes which both absorb in the non-visible (e.g. UV or IR) and have a higher extinction than the dyes absorbing in the visible, then lambda max for the mixture.will occur in the UV or IR region.

Increase the concentration of solution and measure the absorbance. Absorbance may be at near  UV region. Take UP spectra at different concentration to find out the effect of dilution.  This gives an idea of color change on dilution. 

Diluting the solution will not help, as the spectrum shape will not change on dilution. Henry is right! When we perceive "colour", it is because some wavelength range is absorbed stronger than the others. You don't need a maximum to have this effect. Maximum might be in UV, still the flange going to the visible part creates a colour...

Apart from this, it is always better to show the spectrum here, instead of only describing it!

Your solution probably a kit for some compound, looks very concentrated and probably cloudy. A solution should be homogeneous and transparent. The red yyou have is a compound color with some interference like a red brick combination. For sure the red  is present and you will read heavy absorbance at 515 nm that corresponds to red band. If this was not the main absorption peak there is interference from UV aborption and you should purify your extract before applying the test kit in a plate reader, or try to add known amounts of the compond to the eextract and plot the result in a ABS versus fortified dose

As many scientists answered check first the concentration.

From the formula of your substance you can have an estimate of the wavelength 

You need to post the spectrum from 400 - 800 nm for the readers to see. Make sure to have clean solvent in the reference side. Also, post the concentration of the solution.