ds2 (3) poos himself only uses potty/toilet about 1 in 3 poos

[springchik]

He hardly ever has wee accidents but he poos his pants a bit too often. I dont understand it tbh because he will at other times shout poo poo and either sit himself on the potty or I will lift him on the toilet so he is capable and does understand! Yesterday it was 2 poo accidents in one day (morning and afternoons) 3 days previously he pooed in his potty in the morning but pooed his pants in the afternoon. If I tell him off (know you shouldnt) he doesnt then poo at all for 3-5 days!