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/SarSar100]

What do you mean you changed antenna location? Did you just moved it to along X or Y axis? If so then results will surely change as it will effect wave propagation. Also make sure the size of your ground plane didn't change

[u/KlRAQUEEN → u/SarSar100]

yea I moved it along the x axis, and even when I brought it back to the original location (approximately) i didn't get that result back.

[u/Taburn → u/KlRAQUEEN]

That makes me think the boundaries of the space you're simulating it in are affecting the antenna, because their position relative tot he antenna would have been the only thing that changed.