Good morning Stevehowlett,
Thank you for your inquiry. You have made an excellent observation. The bottom tail (thickness) is tied to a bunch of factors but we some of the more common concepts include the following: The bottom tail will typically be thicker with a harder bottom composition than with mud or sand. If you take all the other factors out of it this is a pretty consistent. Please see the photo below, you will notice the bottom tail changes this would indicate a harder bottom moving towards a softer bottom.
Lower frequencies tend to yield a thicker the bottom tail than higher frequencies. You already observed this, we also know that typically the wider the beam angle the thicker the bottom tail. Which works together with frequency, typically the lower the frequency the wider the beam angle hence a thicker bottom. Please see photo below. Comparing low and high frequency and narrow vs. wide beam angle with a fixed frequency.
The "q" rating also can have an impact on the thickness of the bottom tail. It has been our experience that a higher "q" transducer can show a different view of the bottom tail. Please see the photo below. This photo shows two different transducers with the same frequency, same output power, different beam angles but not significantly different, and the same bottom. You will notice the transducer on the right shows the bottom tail much different than the the photo on the left. The photo on the right has a higher "q" value.
As far as seeing fish on your meter at depth. Couple things to keep in mind, the higher frequency has a narrow beam angle. So sometimes you might see fish on the low frequency and not the high do to this. Also the larger the beam angle the larger the targets appear on the screen because the fish stays in the beam angle longer making it appear larger or wider.
Can you also verify the Tx power settings, the Pulse Repetition Rate, and the TVG Depth and TVG Level Settings for both the high and low frequencies?
Lastly we have done a sweep test on that transducer please see the photo below. You will see in the photo what frequency responded with the strongest return echos.
Kind Regards
C-Bass