Ursine strips can only show you ketones that get wasted. Not ones you are using and not the ones in your blood or breath. Breath is most accurate to show what you’re using.
In addition, you don’t even need to be in Ketosis to lose weight. Having ketones doesn’t mean you are burning body fat. Someone can test high in ketones because they are overeating fat to the detriment of body fat loss. Think Jimmy Moore and his weight gain over the years.
There is ALWAYS more dramatic weight loss in the beginning than later on. You can also lose body fat without actually losing any weight at all. I’ve gained and maintained 4-5 pounds over the last 3 years but have dropped 3 pant sizes in that same time. It could be frustrating because I still have fat to lose and am still 10-15 pounds from the goal weight I chose for myself. But I’m at th raise clothes I was hoping for. It’s still hard to totally celebrate that. It’s our nature to be constantly disappointed I think
If you truly begin to gain actual weight and not just water fluctuations, you’ll need to consider how much you’re eating. Are you drinking any calories? Are you doing any natural fasting, like pushing breakfast out a few hours or anything? Are you snacking? Are you adding a stick of butter to everything you eat?
Lack of urine tested ketones isn’t the problem.

If you’re losing don’t worry about the strips. I’d be more concerned that you’re not losing. If it were me I’d look for hidden carbs and cut back on “dirty keto” foods. See if it helps.

There is one theory about pee strips that says once you are fat adapted and your body adjusts to burning fat and producing ketones, you no longer have any ketones to waste in your pee, so the pee strips don't show any. Doesn't necessarily mean you are not in ketosis. Keep calm and Keto on! you are doing fine.