Radar level transmitter problem

I am facing a problem with Sitrans lr250 horn radar transmitter.
Its reading changes very slowly with tank filling (results in liquid overflow).
The thing is that if I changed any setting, like response speed, it reads accurately but then takes very long time to read actual level.
What could be the problem?
This is a common problem. The measurement is only suited to ideal fluids with splashing, foaming, or changing reflectivities. You need an alternate measurement.
An LR250 should read the echo from a high dielectric liquid like water.

A couple things come to mind.

The LR250 is designed for liquid level measurements. The LR260 is designed for solids level measurements. Are you sure you have an LR250? Does the stainless steel nameplate say SITRANS LR250 on it?

The Fast Response Rate is 10m/minute. If the rate of increase is faster than that, then the LR250 is not the right instrument.

The LR250 had a selection for MATERIAL, LIQUID or LIQUID LO DK. Water is not low dK (dielectric). Choosing LOW DK will slow the reporting of level values to allow for more 'shots' to be processed.

Installation is crucial. The manual has recommendations with illustrations on where not to mount the transmitter. Mounting in those areas will create issues with reading echoes and can result in slow processing. The polarization axis plays a role when the transmitter is close to tank wall. Stilling well/pipes create issues with echo return. Poor installation/mounting can create bad echoes that lower the echo quality.
Foaming presents problems in some services.

If you get good reading with reduced filling rate, then you'll have to reduce the rate of filling.