Sounds like your guitar might need to be intonated. Try this: tune your open strings to a tuner, then check the tuning at the 12th fret. If the pitches at the 12th fret are out you need to adjust the intonation.  All guitarists should be able to do this....here's a video on how to do it.

https://www.musiclessons.com/youtube/share/?v=N1QYbpAx-pE

If the intonation is on, it could be one of these things:

Poor quality tuning machines could be slipping. Probably not the problem unless the guitar is total garbage.

Changes in temperature and or humidity. Some guitars necks are slightly more susceptible to changes than others.

Improperly cut nut. If the string slots in the nut are a bit tight, a string can bind in the slot. You will usually hear a "ping" when the string suddenly moves in the slot as you are tuning. A repairman can fix this in minutes. you can also add a bit of graphite to the nut to help the strings move smoothly. ( I use a pencil...just draw a bit in the slots)

String post slippage. Depending on how you attach your strings to the posts, they could be slipping there as you tune them. Here's a good video of how to properly change strings to avoid slipping:

https://www.musiclessons.com/youtube/share/?v=4CtqfHmssx8

this happens all the time to me and it may be where you keep knocking the end of the guitar and the tuning pegs are moving slightly.

If you have a problem with your guitar you can change it. Buy best sounding acoustic guitar with best quality.If you want to know about more branded and  stylish Acoustic guitars which is used by great musician visit this site.

http://buyacousticguitaronline.com/