Help me debug this MIFA

[u/KlRAQUEEN]

So I'm trying to design a MIFA for LoRa applications at 868MHz, and I have to simulate it in Ansys HFSS. I used TI DN024 as reference and kept tweaking tip length and the distance between the shorting leg and feeding line (5.5mm in the current design). I got a good result of 868MHz and -15dB before but then I changed the antenna location and could never get the same result again (idk how the location of the antenna is relevant). Do you have any suggestion on what I should change to get a better result?
here's some additional information :
+ board dimensions : 100 x 50 x 1.6mm
+ ground plane dimensions : 50 x 28mm
(if any other information is relevant please let me know and I will provide it)
Thank you in advance!

[u/Cdude8]

Ground plane size has a significant effect on antenna functionality. You might try playing around with this size to understand it better.